What exactly is a dynasty?

As I understand it, it’s an aristocratic family that has been ruling a country for generations, right? Something like Windsors, or Saxony-Coburg-Gotha, that have been ruling England for quite a while. Is it that, or is there more a series of monarchs have to fulfil to qualify as a dynasty apart from being related?

If that’s the correct (and only) definition of “dynasty,” I’m wondering about the term popping up so often when reading about Egyptian history. When dates are given, it is usually stated the thing happened in the 11th Dynasty, or whatever. Probably they’re really just families of rulers, and the large number of dynasties is because some got extinct or was overthrown or whatever, and another family succeeded. Or OTOH, probably all those dynasties are just an arbitrary way of dividing Egypt’s millennia of history into periods, without relevance of who the ruler descends from?

The term “dynasty”, as applied to ancient Egypt, has the conventional meaning of an interval of time during which a particular family line ruled. That is, this is what it usually means.

The record concerning some intervals of ancient Egyptian history is extremely sketchy, and our knowledge about many dynasties is tentative and vague. Pharoahs can be assigned to the Late, Middle and Early Kingdoms, but beyond knowing the order in which they ruled, information about how long a particular pharoah reigned, or precisely when, is often a matter of educated guesswork.

When it is said that Tutankhamen was the last pharaoh of the 14th Dynasty, we are stating that it is known that he succeeded a relative (though there is some dispute as to how they were related), and that his successor was the founder of a new line.
But while we know that this new line, called the 15th dynasty, came immediately after King Tut’s reign, we don’t know, really, what numbers should truly be assigned to the dynasties we call the 14th and 15th.

This is because information about long intervals in Egyptian history from before this time is very incomplete. The system used for numbering the dynasties was developed by an ancient Greek historian who lived long after most of these pharoahs ruled. To accomodate a bunch of extra names for rulers for which he could not properly account, the Greek historian (unfortunately, his name escapes me now), cobbled together a hypothetical dynasty (IIRC, the 7th), in which a long succession of pharoahs ruled, each for exactly one day.

This page, http://www.ancient-egypt.org/glossary/people/manetho.html says

A dynasty is a succession of rulers of the same line of descent. The term is popularly used to refer to any family or group that has done anything for two or more generations (the “generations,” in some cases, being as short as a year). In Egyptology, “dynasty” is used in its strict sense, and also to delineate the time during which a given dynasty ruled.