Psycholinguistically, a lemma is an abstract conceptual form of a word. However from a lexicographic perspective, the lemma is merely the aorist or canonical form of a word.
In English, the lemma of a verb is the 3rd person singular form; for a noun, it's the singular form.
However for multi-word expression, we might need to use the plural form of a noun within the expression, e.g. "kick_one's_heels". Even if we don't consider multi-word expression to constitute a lemma, we have other inconsistencies of the criteria to give lemma status to a word, e.g.
What is a notion of lemma? How is it scientifically defined? Citations will be much appreciated.