发芽（英语：Germination，亦称为萌发）是指植物从种子中发出及真菌从孢子中发出的过程，之后它们开始生长。发芽最常见例子就是对被子植物或裸子植物的种子进行催芽而形成幼苗。然而，从孢子而出的孢苗生长亦称为发芽，例如从真菌孢子中长出菌丝。更广义上，发芽可以指任意一个从小的存在或生殖细胞展开扩大成为更大存在的过程。

Germination is the process by which a plant grows from a seed. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. In addition, the growth of a sporeling from a spore, such as the growth of hyphae from fungal spores, is also germination. Thus, in a general sense, germination can be thought of as anything expanding into greater being from a small existence or germ.