An isolex is a sort of isogloss. To save you having to follow the link, an isogloss is a line surrounding an area with a common linguistic feature, and an isolex is a line on a map surrounding an area where a particular word is used.
For example, the isolex for the word bargain, in the sense of a plastic beaker with a lid containing a small opening to admit a straw, would tightly surround my current house. (That's a bargain, I said long ago to my two very small daughters, when I spotted a cheap one on a shop shelf. And so it always was, as far as they were concerned.)
Your own house could probably have its own isolex drawn round it - or, if not your house, one might surround the environs of your family. (I doubt, for example, that many other people call a mincemeat turnover a mysteron, as my family does.)
Still, now we know we can draw a line called an isolex round the area that encompasses our families' use of our special words for stuffed rabbits, or trips to the loo, or whatever, we can feel rather proud, and rather clever, when we use it.
Word To Use Today: a word that has a very local isolex. The iso- bit of this word comes from the Greek isos, which means equal, and the lex bit cones from the Greek word lexis, which means word.