Definition of LEXICAL

: of or relating to words or the vocabulary of a language as distinguished from its grammar and construction

Source: http://www.merriam-webster.com/dictionary/lexical

Function words (or grammatical words or synsemantic words or structure-class words) are words that have little lexical meaning or have ambiguous meaning, but instead serve to express grammatical relationships with other words within a sentence, or specify the attitude or mood of the speaker. They signal the structural relationships that words have to one another and are the glue that holds sentences together. Thus, they serve as important elements to the structures of sentences.

Words that are not function words are called content words (or open class words or lexical words or autosemantic words): these include nouns, verbs, adjectives, and most adverbs, although some adverbs are function words (e.g., then and why). Dictionaries define the specific meanings of content words, but can only describe the general usages of function words. By contrast, grammars describe the use of function words in detail, but treat lexical words in general terms only.

Source: http://en.wikipedia.org/wiki/Function_word