1. A bristle or bristle-like appendage, especially in an invertebrate.
2. The stalk of a moss or liverwort capsule.
The plural is 'setae'. More details on Wikipedia, of course.
I ran across this word while researching the caterpillar I saw in the park today.
Picture below (not mine - snagged from internet) - it's a caterpillar of an American dagger moth.
The yellow hairs and the longer black hairs are all setae.
On contact, the setae of some caterpillars can break off and embed themselves in the skin, causing welts. Here's an article; tape can be used to remove the setae. (Yes, I did let the caterpillar briefly walk on me, but I didn't touch the hairs - no welts so far.
Etymology: from the Latin word for 'bristle'.