Lemmas have special significance in highly inflected languages such as Arabic, Turkish, and Russian.
In European languages with grammatical gender, the citation form of regular adjectives and nouns is usually the masculine singular.
For many languages, the citation form of a verb is the infinitive: French aller, German gehen, Hindustani जाना/جانا, Spanish ir.
For Latin, Ancient Greek, Modern Greek, and Bulgarian, the first person singular present tense is traditionally used, but some modern dictionaries use the infinitive instead (except for Bulgarian, which lacks infinitives; for contracted verbs in Ancient Greek, an uncontracted first person singular present tense is used to reveal the contract vowel: φιλέω philéō for φιλῶ philō "I love" [implying affection], ἀγαπάω agapáō for ἀγαπῶ agapō "I love" [implying regard]).
For Arabic the third-person singular masculine of the past/perfect tense is the least-marked form and is used for entries in modern dictionaries.
This is similar to Hebrew, which also uses the third-person singular masculine perfect form, e.g. ברא bara' create, כפר kaphar deny.
In Tamil, an agglutinative language, the verb stem (which is also the imperative form - the least marked one) is often cited, e.g., இரு In Irish, words are highly inflected by case (genitive, nominative, dative and vocative) and by their place within a sentence because of initial mutations.
Some phrases are cited in a sort of lemma: Carthago delenda est (literally, "Carthage must be destroyed") is a common way of citing Cato, but what he said was nearer to censeo Carthaginem esse delendam ("I hold Carthage to be in need of destruction").
A word may have different pronunciations, depending on its phonetic environment (the neighbouring sounds) or on the degree of stress in a sentence.
An example of the latter is the weak and strong forms of certain English function words like some and but (pronounced /sʌm/, /bʌt/ when stressed but /s(ə)m/, /bət/ when unstressed).
The OED and the DWB, for instance, include exhaustive historical reviews and exact citations from source documents not usually found in standard dictionaries.
As for nouns, adjectives are listed in the nominative singular (for languages that inflect for grammatical case or number).
Likewise, for Ancient Greek the traditional headword is the first-person singular λέγω (legō), and not the infinitive λέγειν (legein).