Féminin vs. femelle

When discussing gender-related terms in French, 'féminin' and 'femelle' both refer to the female gender, but they are used in different contexts.

Féminin

Relating to women or girls; feminine. Used to describe attributes or nouns that are associated with women or the female gender in cultural, societal, or grammatical contexts.
Des vêtements féminins.
(Women's clothing.)
La beauté féminine est célébrée dans de nombreuses cultures.
(Feminine beauty is celebrated in many cultures.)
En français, les noms et adjectifs ont un genre masculin ou féminin.
(In French, nouns and adjectives have either a masculine or feminine gender.)

Femelle

Biological term referring to the female member of a species; used mostly for animals and rarely for human beings due to its impersonal and dehumanizing connotation when referring to women.
La lionne est la femelle du lion.
(The lioness is the female of the lion.)
Chez les oiseaux, cest souvent la femelle qui couve les œufs.
(In birds, it is often the female that incubates the eggs.)
Il est incorrect et offensant d'utiliser 'femelle' pour parler d'une femme.
(It is incorrect and offensive to use 'femelle' when talking about a woman.)

Summary

In summary, 'féminin' emphasizes traits and qualities culturally attributed to women or grammatical designations within language, while 'femelle' focuses on biological distinction among animal species and is infrequently and inappropriately used regarding human beings due to its dismissive tone. Understanding the context in which these terms are properly applied is key to their correct usage in the French language.