noun
mold; mildew
Refers to the fuzzy or powdery growth of fungi on organic matter, especially in damp conditions.
パンにカビが生えた。
Mold grew on the bread.
浴室のカビを掃除する。
I clean the mold in the bathroom.
菌 is a broader term for bacteria, fungi, and germs, while カビ specifically refers to visible mold or mildew.
The origin is uncertain, but it may be related to the verb かぶる (to cover) or かびる (to become moldy). The kanji 黴 is a Chinese character for mold.