Fat
Grease, Blubber
noun
If you add fat to fat, what do you get? You get fat, obviously. Come on now, this isn't rocket science.
脂肪 specifically refers to the solid fat found inside plants and animals. In casual conversation, it's often used when talking about someone's excess body fat.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
太ももの脂肪を落としたいんです。
I want to lose the fat from my thighs.