A word, in mathematical terminology, is a sequence of letters from some finite alphabet, such as "aabaaab" or "LLsLLLs." In music theory, they are often used to represent an abstract scale template with some specified pattern of step sizes, such as when working with MOS scales.
For example, the word "aabaaab" can be thought of as representing a scale template where "a" represents one kind of step, and "b" represents a different kind of step. It is often useful to specify the relative sizes of the steps as well, leading to the two related words "LLsLLLs" and "ssLsssL," where L means "large" and s means "small." Of these, LLsLLLs basically corresponds to the diatonic scale, and ssLsssL the anti-diatonic scale; aabaaab can be thought of a more generic word incorporating both into a single template.
There is some terminological subtlety regarding what the word "scale" means, and thus some semantic subtlety in what a "word" is said to represent. Often, authors treat the term "scale" as an actual collection of real notes or intervals with a precise tuning, as in (0, 200, 400, 500, 700, 900, 1100, 1200) for the 12-EDO diatonic scale. In this sense, a word like LLsLLLs can be thought of as representing a family of these precisely-tuned scales matching the step pattern of the word. On the other hand, often one refers to "the diatonic scale" as an entity in its own right, agnostic to any particular tuning, which can be tuned in different ways; in this sense that LLsLLLs word represents the diatonic scale, treated as a single abstract entity.
A munit is a word with a specified size for the outermost interval.