Anything that sticks or juts out from a surface is a protrusion. When you're rock climbing, every protrusion is a possible handle to grab or step to stand on.
In medicine, most protrusions are evidence of some disorder or illness, like a bulging protrusion in your belly. Most other protrusions, though, are perfectly normal: your nose is a protrusion in the middle of your face, and a spire can be described as a protrusion on the top of a church. The Latin source of protrusion is protrudere, "to push out."