Stands for
internal pro
tein - or
internal prot
ein, come to think of it. Internal to another
protein, that is. One that is
spliced out of the outer protein (the
extein) to produce a
mature host protein and a free intein.
This might seem a little bizarre, but that's biology for you. It is as if a 'virus-like' protein is embedded in a larger one. It is more precise to say that a protein does some autoprocessing on an internal, rather than a terminal, sequence. The obvious analogy is with introns in genes, although the intein gene is probably not an intron!
There is a database of inteins at:
http://www.neb.com/neb/inteins.html.