Oooooh boy...I don't have a name for it but I had one of these once. I went to the doc and he said it was probably an ingrown hair or just a clogged pore gone nuts. He explained that it just keeps filling up with puss or whatever, gunk...and if it doesn't disssapte on it's own it can get hard under the skin.
He then asked me if I wanted to have him stick a needle in and pop it out, or let it be and see if it would go away on its own. I was like, "Let it be!" But he recommended that I just get it done since he didn't think it would go away without help (it had been there for like 6 weeks).
Well, owie. it did hurt pretty bad. He asked me if I wanted to see "it" after it popped out, and I was like NO! But now I wish I wouldv'e looked.
FYI, that same cavity filled up with stuff (I guess...the lump came back in other words) after I gave birth and I was so freaked out about going back...I prayed that it would go away on it's own and it did.