Descendent was originally an adjective meaning either (1) moving downward or (2) descended from an ancestor. But it is almost never used this way. Here’s a rare example of the word used in its conventional sense:
But, his lunch shakes were directly descendent from his fathers’: spinach, lentils, whole wheat bread, olive oil, avocado. [Ian and Julia (now offline)]
Descendant is a noun meaning someone or something derived from an earlier person, animal, or thing—for example:
Burgess was the descendant of an old merchant family that had suffered reversals. [Boston Herald]
” Well I’ll be there,” smiled the Terenure man who is a descendant of a pure Kerry sporting bloodline. [Irish Times]
In current English, however, descendant usually covers descendent‘s territory as well as its own. In fact, correct use of descendent according to its conventional sense is so rare that we should probably consider it a dead word. It may have been useful in the past, but people apparently don’t find it useful today.
