Simply put, linguistics is the systematic study and investigation (some would even say the science) of language. Although some linguists speak two or three or more languages, most of the linguists I know only speak English and, furthermore, of those linguists who do happen to know multiple languages, their polyglot gift doesn’t usually have much to do with the work they do as linguists.
Linguists study language, plain and simple, in all its glorious parts. If a linguist tells you he’s a phonetician (like me) or a phonologist, that means he studies the sounds of speech — the consonants and vowels that make the basic parts of any word. Moving up to slightly larger units, morphologists study morphemes — parts of words slightly bigger than sounds, but not always as big as a word — things like suffixes and prefixes (more on that, here). Looking at words themselves we have lexicographers (the people who write dictionaries) while linguists who investigate how the meanings of words interact with each other are called semanticists. Linguists who tend to study language at the level of whole phrases and sentences are called syntacticians (actually, syntacticians study syntax, which isn’t quite “studying sentences”, but you get the idea). Then, at the “top” of this language pyramid, we find the linguists who study large chunks of sentences… we call these people conversation analysts.
And then there are the linguists who don’t study the parts of language, per se, but how these parts fit together and how language fits into the larger world around us. Sociolinguists investigate how language interacts with society, documentary linguists study and document minority and indigenous languages, computational linguists study how language and computers work together (more or less), historical linguists study the way languages change and evolve over time, and when you have a linguist who likes to do her research on how people talk about language (talk about talk), you’re probably dealing with a discourse analyst.
All of these kinds of people, all of these kinds of research, are a part of linguistics (and there’s even more than that, too!). Knowing two or more languages? Nope. Not required at all to be a linguist.
So, the next time you’re at a cocktail party, or a wedding reception, or sitting on a plane and you find yourself talking to a linguist, ask them about their research (definitely!), but please don’t ask them how many languages they speak. That would be like asking a wedding planner how many spouses they have.