I believe a direct ancestor is someone without whom you would not be here. All your other ancestors are indirect.

When the term ancestor is used correctly, there is no such thing as an "indirect" ancestor. For example, your grandfather's brother is not your ancestor based solely on that relationship.

Direct-line ancestor is someone from whom you descend in a direct line, parent to child, grandparent, great-grandparent, etc.

One that only has Fathers and Mothers involved - that is for example; your mother's father's father's mother. But doesnot include spouse or sibling links.