The Macmillan dictionary says the word trousers is "mainly" British, which implies that it's not entirely British and Americans also use it. I have read a few novels in which American writers use the word trousers not pants--the most recent being Alex Kava's "Stranded". Do many Americans wear trousers instead of pants? Is it a regional variation or just a less common synonym that every American sometimes uses? Or might it be a particular type of pants that are called in this way?
In the US you will almost never hear, "trousers", if at all, outside of certain written texts (though anyone will understand what you mean). It's just exceedingly rare to hear a native US citizen say something like, "Wow, it's so cold out, I'm glad I wore trousers today.". Extremely rare.
US: pants = vernacular for a garment covering your entire legs (such as jeans, slacks, sweats, etc... which are just more specific forms of the same thing).
UK: trousers = vernacular for a garment covering your entire legs (such as jeans, slacks, sweats, etc... which are just more specific forms of the same thing).
US AND UK: underpants/underwear = the garments you wear underneath your main garments (such as pants or trousers or shorts) to cover your private areas and keep things clean.
Myself. I'm a US citizen from central Texas in my 40s. This is based on 40 years of experience listening to people, reading, and watching TV. I have rarely heard this usage outside academic speeches, papers, and some other forms of writing (i.e. fictional works). In hearing parents, friends, co-workers, an ex-wife, my children, their friends, acquaintances, and even strangers, I have rarely ever heard the word "trousers" in everyday vernacular.
It is somewhat anecdotal, I'll give you that much, but it's also so obvious that I would venture more than 99% of native Americans would not use trousers in everyday speech. The word pants is without question the most dominant form of describing a garment that covers one's legs in the US (or the specific forms of pants: slacks, jeans, sweats, etc.). This does not mean it's never used, or that Americans don't know what trousers are; that would be an absurd notion and should not be the idea gathered from this answer.
As a small piece of anecdotal evidence, in the American TV show 'The Wire' (season 3 episode 1, 2004), the character 'Bubbles', a heroin addict in Baltimore, is complaining that he was forced to give up his pants after damaging a drug dealer's car. He says:
"[I] lost my trousers man, my fucking trousers".
Perhaps this suggests that the word is used regionally- there certainly is no suggestion that he is referring to formal wear.