something extremely tiny
Figurative use, often in comparisons with ほど or のよう, emphasizing extreme smallness.
He has not even a poppy seed's worth of heart (i.e., he is utterly heartless).
This bread has a lot of poppy seeds in it.
Standard kanji spelling for 'poppy seed' and the figurative tiny-thing sense.
Mixed kanji-katakana spelling, sometimes used for clarity or emphasis.
Compound of 芥子 (keshi, 'poppy') and 粒 (tsubu, 'grain/seed'). The figurative sense derives from the small size of poppy seeds.