General: in heat
Images or animations depicting female characters undergoing the physical condition known as estrus (BrE: oestrus), colloquially known as being "in heat".
If the lady looks aroused in a more human way, you probably ought to be using horny instead, or some other tag. There's overlap and punning all the same, so carry on using the heat tag if you need to - but it makes sense to reserve this tag for something very specific.
Because estrus is kind of a weird state to the human eye, and supposedly more intensely experienced first hand (and more obvious to dumb drooling males), cartoonists and comics authors often represent it with little symbols or wavy lines emanating from the creature's hindquarters.
