A "fat cell" (that's its literal meaning). One of the cells in the body that stores fat. They are the cells of adipose tissue, such as the subcutaneous fat of the body (under the skin).
"Fat" sounds passive but it is actually a biochemically active cell, in that it is part of the endocrine system and produces hormones. One is the molecule leptin, which affects body weight. Another is the effector derived from the agouti gene, which affects coat colour in some animals.
They are generally largish, round, and white in appearance, as you would expect a fat cell to be. They are connected to the muscle fibre and connective tissue of a body.