noun
Literal meaning: the small seed of the poppy plant, used in cooking and baking.
See also: けしの実
このパンには芥子粒がたくさん入っている。
This bread has a lot of poppy seeds in it.
noun
something extremely tiny
Figurative use, often in comparisons with ほど or のよう, emphasizing extreme smallness.
彼の心は芥子粒ほどもない。
He has not even a poppy seed's worth of heart (i.e., he is utterly heartless).
Also means 'poppy seed', but 芥子粒 is more common in figurative expressions for tiny things.
Compound of 芥子 (keshi, 'poppy') and 粒 (tsubu, 'grain/seed'). The figurative sense derives from the small size of poppy seeds.