1. lemma - Noun
2. Lemma - Proper noun
A preliminary or auxiliary proposition demonstrated or accepted for immediate use in the demonstration of some other proposition, as in mathematics or logic.
Source: Webster's dictionaryA particle moving with a uniform velocity would be denoted by Wallis by the formula s = vt, ...while previous writers would have denoted the same relation by stating what is equivalent to the proposition s1 : s2 = v1t1 : v2t2 (see e.g. Newton's Principia, bk. I. sect. I., lemma 10 or 11). John Wallis
According to his lemma, a group of four manuscripts including Codex Monacensis 1086 are copies directly from the original. Source: Internet
Advanced mathematical proofs like Siegel's lemma build upon this more general concept. Source: Internet
A well-known joke alluding to this equivalency (which may defy human intuition) is attributed to Jerry Bona : "The Axiom of Choice is obviously true, the well-ordering principle obviously false, and who can tell about Zorn's lemma?" citation. Source: Internet
A related result, sometimes called the second Borel–Cantelli lemma, is a partial converse of the first Borel–Cantelli lemma. Source: Internet
Both these results can be proved by the tube lemma which is used in the proof that a product of finitely many compact spaces is compact. Source: Internet