This is a question whose answer surprised me when I first heard it many years ago. I was well aware that many words had more than one meaning, and could think of a few obvious examples. Still, the answer, though it certainly has a few obvious different meanings, was not what I was expecting.
The word is set.
Plain, simple, unassuming set. Seems odd, doesn’t it? Of course straight away you can probably think of a few different meanings for it. It can be a verb meaning to arrange or put in order. It can be a noun referring to a united group of entities, either in an abstract mathematical sense, or the more mundane sense of a chemistry set. And tennis matches are divided into sets. And you can set something aside, or down.
So yes, a few different meanings, but it probably still sounds surprising that the Second Edition of the Oxford English Dictionary contains 430 definitions for set, and most other dictionaries will have about the same number. Have a look for yourself if you have one to hand.
There are a few main reasons set has so many definitions. First, it’s a hot verb, meaning it doesn’t have one simple main definition, but can be used with many different meanings, depending on the context. Second, this also means it’s used a lot as a phrasal verb, e.g. set down, set out, set out to, set up, which all count as distinct definitions in the dictionary. Set also has the advantage of its ability to be used as a number of different types of words. It can, for example, be a verb (set your watch), a noun (a mathematical set), and an adjective (a set number of people).
Still, don’t get too used to set claiming the top spot. In fact, if you look in some modern editions of dictionaries, you might find that run and put take up more space. Officially though, the Second Edition of the OED is taken to be the definitive authority on words and their meanings. The problem is, it was published in 1989, and obviously things have evolved linguistically since then. Never fear though, the third edition is being worked on right now, and we can expect that once it’s published, set will have been replaced as the word with the most definitions. They’ve been working on it since 2000 and are about halfway through, so eh, not too long to wait now!