I wonder what differences are between usage of slacks, pants, and trousers? Their meanings seem the same by looking up Google’s Internet dictionary and Wikipedia.
An excert from here:
Slacks implies pants of certain materials which are not part of a suit (jeans are not slacks, and you would not refer to the pair of trousers that came with a suit as "slacks".) It is also more common to use "slacks" to refer to pants worn by women, while men would wear "trousers". Meanwhile, "pants" could refer to slacks, or trousers, or jeans, or just about any form of two-legged outer garment for the lower body.
Note that in AE, "pants" by itself is never understood to mean underwear of any kind, and must be altered in some form (either as "underpants" or as "panties") to have that meaning.
Bill: What's this I hear that the boss walked into your office while you were changing your clothes and caught you in your underwear? Tom: No, but she nearly caught me in my underwear; luckily, I had just put my pants on.
This is an AE perspective but, I would say that trousers and pants are synonyms. With both being any outer garment that covers both legs separately and goes from waist to ankles. Technically slacks is also a synonym, but the informal definition I most frequently hear is that slacks = dress pants. I.E. Pants that you might wear if you were trying to look nice.
I know the question is tagged AmE - but that wasn't in the original, and hasn't (yet) been confirmed by OP, so I don't see anything wrong with answering from a BrE perspective.
In the UK, pants almost always means underpants. And as OED says, slacks are loosely-cut trousers for informal wear, esp. those worn by women.
I think for many Americans, pants and trousers are effectively synonyms. But so far as I'm aware, "esp. those worn by women" tends to apply to slacks on both sides of the Atlantic.
An AE perspective:
pants - The broadest term used for any full length two legged covering, male or female; covers both hard work clothes (jeans), casual (chinos, ducks, khakis, etc.) and dress outfits (suits, office wear, evening wear, even tuxedos). This is probably the most common term in general use. (See ngram below which is probably book references only)
slacks - The next fairly broad term, covering all but work clothes or jeans. Women's dressier pants are often called slacks. Probably least used overall and more for women's clothing.
trousers - Usually only used for dressier clothes, such as suit pants or finer pants often worn with a sports jacket, blazer or a dressier shirt. Usually reserved for men's clothing, with the possible exception of women's pants suits.
A good dictionary is more useful for comparing regional usage and subtleties of difference than Google or Wikipedia. From Macmillan Dictionary:¹
a piece of clothing covering the body from the waist to the feet, divided into separate parts for each leg and worn by both men and women
(American edition) a piece of clothing that covers your body from your waist to your ankles and has a separate part for each leg
(British edition) a piece of underwear that covers the part of the body from the waist to the top of the legs
(British edition). trousers, especially ones that are worn for informal occasions
(American edition) pants, especially ones that are worn for informal occasions
Well in Britain,
1) Slacks - (Clothing & Fashion) informal trousers worn by both sexes (The Free Dictionary)
2) Pants - inadequate, displeasing, or of poor quality. Possible origin: underwear, called "pants" in Britain. (Online Slang Dictionary)
3) Trousers - (Clothing & Fashion) a garment shaped to cover the body from the waist to the ankles or knees with separate tube-shaped sections for both legs. (The Free Dictionary)
I understand you want to know what the differences are in US-English, but that is the definition in UK-English.
In South Africa, it is the same as USA - English. Where we would call trousers as formal long pants. i.e.
Pants = shorts / baggies, etc.
Trousers = formal long pants; for weddings, etc.
I have never heard of slacks, but guessing they would be loose clothing.
The real question that keeps popping up is the definition of "slacks". It is a word that was used in the US clothing trade to describe what might be called "dress pants" in the US: a type of garment that looks good with a blazer. This garment could be khakis, gabardine, some kind of cloth that holds a crease. "Dockers" would be an example of such a garment. I wouldn't imagine flannel trousers being called slacks, as they do not hold a crease. But that's just a personal impression.
Think of a man's suit, the type of cloth; now imagine just the trousers, and that there is no matching jacket: those would be slacks.
Delving further into the question of pants/slacks/trousers/jeans, all of these are pants of some kind, at least in US vernacular. Trousers is generally a term for pants that have a matching jacket, though the term "pants" is used as well. Slacks, as I said above, are "dressy" without being formal, and do not have a matching jacket. Jeans, on the other hand, are jeans. Denim riveted if truly authentic, exterior back pockets ... perhaps also with a small watch fob in the front right pocket; no crease down the front. Although garments can be cut like jeans, sewn like jeans and styled like jeans, if they are not made of cotton denim, they are not truly jeans.
One final note on "slacks" ... since there were many German Jewish immigrants in the garment trade a hundred years ago, I wonder whether the term does not come from Yiddish or German, just as "lox" (smoked salmon) comes from "Lachs" (German for salmon). That's one for a German linguist to pursue.
The usage of the words "slacks" and "pants" to mean the same thing as the word trousers, seems to be from American English. I have heard these used by Americans. I have not heard those words used by British people. They normally use the word trousers and sometimes, some slang/informal words for them, like "strides".
This page lists the word "slacks" as another word for pants (in the trousers meaning): http://www.h2g2.com/approved_entry/A129647
I am from the North West of England and I am very American on this subject, pants are not underware. Trousers are smart and formal, you would not call the lower half of your addias track suit "jogging trousers"
If its raining I put on waterproof pants. (another word overpants) If i am going out on a Friday night to a club i will ware trousers and a smart shirt.
I do not use the term slacks.
Seems to depend on where you are from in the U.S., as well. In the Midwest, or at least the part where I am from, slacks refers to casual dress pants — what you'd wear to work, out for the night, to a dressy occasion where you don't want to look stiff, but do want to look good. Blue jeans don't fit this word. Jeans are jeans. Pants is a universal term. You subdivide, by using slacks etc., in order to define exactly what style you mean.
In UK English,
trousers worn by women. This is by contrast with the other form of similar leg covering garment worn by women:
Trousers usually implies that the wearer is male.
In UK English,
pants is a less formal word for
trousers. It doesn't usually mean
underpants, although if you
wet your pants, I suppose this might mean
In recent years,
pants has been used jocosely to mean useless/rotten/rubbish.
Trou´sers n. pl. 1. A garment worn by men and boys, extending from the waist to the knee or to the ankle, and covering each leg separately.
Trousers or "pants" can either be synonymous with pants or have a more formal connotation. Therefore, all trousers are pants, but not vice-versa: jeans are pants but not trousers, while slacks (formal pants) are both. This definition is consistent with other languages such as the Spanish pantalones, which is contrasted with pantalones cortos (shorts, or literally "short pants").
I have always thought of trousers as part of a suit or uniform. Slacks look the same as suit trousers because they are made of more formal material, but are purchased separately. They would be worn alone or with a sportcoat. Pants are everything else, corderoys, chinos, jeans, and other less formal styles.