Wiktionary
n. (context computer science English) The conversion of a stream of characters to a stream of meaningful tokens; normally to simplify parsing.
Wikipedia
In computer science, lexical analysis is the process of converting a sequence of characters (such as in a computer program or web page) into a sequence of tokens (strings with an identified "meaning"). A program that performs lexical analysis may be called a lexer, tokenizer, or scanner (though "scanner" is also used to refer to the first stage of a lexer). Such a lexer is generally combined with a parser, which together analyze the syntax of programming languages, web pages, and so forth.
Usage examples of "lexical analysis".
His field was lexical analysis, just barely close enough to discourse analysis that he had responded to my voice mail.