What is the difference between fluid and liquid?
I'm thinking of this in the context of drink plenty of fluids.
A fluid can either be a liquid or a gas; as such, fluid is a hypernym of liquid in this regard. In your example
Drink plenty of fluids
it's only idiomatic to use fluids instead of liquids. (I wonder what else could be drunk besides a liquid/fluid, anyway!) You also hear things like:
Your body needs lots of fluids
In day-to-day usage, one will find liquid only used when the state of matter of a substance is in question or important to note. For example:
"How's the brownie mix looking?" "It's still basically liquid!"
When ice-cream is involved, melted is more likely to be used instead of liquid. One may also find liquid used in a financial context:
They think I'm liquid but, man, I'm flat broke!
Fluid just sounds smoother and it rolls off the tongue much easier. That probably explains why it's more commonly used in place of liquid. People would probably use gas if they wanted to be specific. Here are some common idiomatic phrases:
From the above examples, you may notice that fluid tends to be used for a liquid substance that has a specified/particular purpose. Thus, transmission liquid would be unidiomatic, for instance.
Fluid describes a substance that has no fixed shape and changes shape freely based on outside influences. It can describe two states of matter: gas and liquid. To think of it another way, a liquid is a fluid, but a fluid is not necessarily a liquid.
In general usage, "liquid" and "fluid" are synonyms.
In a scientific context they are not.
"Liquid" is a state of matter: solid, liquid, gas, plasma, Bose-Einstein condensate
"Fluid" describes certain properties generally attributed to substances in a liquid state
The degree of fluidity depends on the substance and the prevailing conditions (e.g. temperature and pressure).
Consider water, motor oil and maple syrup. At room temperature, they are all liquids. As the temperature drops, all three liquids get denser and flow more slowly.
At 32° F / 0° C water becomes a solid (ice) and no longer flows. Motor oil and maple syrup continue to flow, but the syrup flows more slowly.
In general usage it has become "less fluid".
As a noun, Merriam-Webster's explicitly defines liquid as "a fluid."
Unless you are writing in a scientific or explicitly academic context, I'd say the two are interchangeable. Note that as adjectives, they do have slightly different meanings.
liquid: existing as or having characteristics of a liquid; especially tending to flow
fluid: subject to change; variable
Additionally, liquid is a state of matter. Not all liquids can be consumed.