"How is the meaning of a word to be described? If one is waiting a lexicon, a tool whose basic function is to indicate meanings of words, how is it done? Today when we have learnt how difficult it is to say even what "meaning" is, we may be unsure. But for lexicographers of the past, from antiquity onwards, an answer was ready to hand: use another word, a word equivalent in meaning . . .
"This is definition by gloss. That is, the offering of another word that is more or less equivalent in meaning in the same language, or, in a bilingual context, a word felt to be equivalent in another language. Such a method not only indicates meaning, but also, in the case of bilingual lexicography, offers a translation. The gloss is the way one can translate the word into one's own language; it is a translation equivalent."
John A. L. Lee, A History of New Testament Lexicography, Studies in Biblical Greek 8, ed. D. A. Carson (New York: Peter Lang, 2003), 15-6.