People who use the term "Direct descendant": what exactly do you mean? Is there some other kind of descendant?

Personally, I think the term "direct descendant" is redundant. You either descend from someone, or you don't. There's no indirect about it. We descend from our ancestors: grandparents, great-grandparents, etc. We don't descend from aunts, uncles and cousins, even though they share ancestors with us. I know there are people who use the term "ancestor" and "descendant" more broadly, and that some lineage societies will accept members who have an "indirect ancestor" who qualifies. And that's fine. But in my own picky little head, I define it more narrowly. An ancestor has "-parent" in the name, and those are the folks we descend from. Our other relatives are just that: relatives.

Actually there is another kind - 'collateral descendant'.

The difference is that while a 'direct descendant' of X is the child of the child of the child [repeat as often as necessary] of X, a 'collateral descendant' is the child of the child of the child [repeat as often as necessary] of a brother or sister of X.

For example, the present Queen of the UK is a collateral descendant of Henry VIII, because her ancestress Margaret Tudor was Henry's sister. But she's not his direct descendant, because none of his children produced any children themselves.

Incidentally, you can be a direct or collateral descendant through male or female lines. If you mean male line only, you need to specify that, e.g. 'direct descendant in the male line'.

Direct descent is exactly than your parents, Grandparents, Great Grandparents. Aunts, Uncles, Cousins Nieces, Nephews are indirect because the lineage is mixed with another family.

I am directly descended from my parents, my grandparents, my great grandparents, my great great grandparents and so on.

I have a collateral descendancy from aunts, uncles and cousins in my ancestors' family tree.

It varies.

When some people say direct descendant they mean just male lines.

yes direct descendent is father, grandfather great grandfather. etc.. linear descendant would be father uncle, etc..

there is a big difference, especially if you can trace your lineage far