One of the individual female reproductive organs in a flower.
A discrete organ in a flower, capable of receiving pollen and producing a fruit.
Sometimes used to refer to a single carpel, or to several carpels fused together.
Latin carpellum, from Greek karpos, fruit.
Latin, pistillum, pestle
The pic is a close-up of a magnolia blossom in my yard a couple weeks ago.
The green bit is the pistil, and I believe the yellow spikelets are individual carpels.
(The paler yellow bits with reddish at the base are the pollen-producing stamens.)
Once pollinated, the pistil becomes a brownish spike full of seeds.