What is inbreeding?

I've put up a bunch of posts relating to inbreeding recently (1, 2, 3, 4). But I haven't really defined it. First, let's stipulate what inbreeding is not: it is not the same as incest. Acts of incest can include individuals who have no blood relationship to each other (e.g., Hamlet). Additionally, there are instances of inbreeding which are not necessarily incestuous. If a population is highly inbred, then individuals who are not relations by social custom may still be so genetically similar to a point where the pairing can not credibly be stated as an outcross. But still, what do I mean? To refresh myself I re-read the section on inbreeding in Hartl & Clark. And I think that helped clarify one implicit assumption which I have which may not be clear to everyone, and I'll get to that. In any case, first, what's the deal with inbreeding? The ...