What is Lineal Descendant?

Legal Definition
A lineal descendant, in legal usage, is a blood relative in the direct line of descent - the children, grandchildren, great-grandchildren, etc. of a person. In a legal procedure sense, lineal descent refers to the acquisition of estate by inheritance from grandparent to parent and parent to child, whereas collateral descent refers to the acquisition of estate or real property by inheritance from sibling to sibling, and cousin to cousin.

Adopted children, for whom adoption statutes create the same rights of heirship as children of the body, come within the meaning of the term "lineal descendants," as used in a statute providing for the non-lapse of a devise where the devisee predeceases the testator but leaves lineal descendants.

Among Native American tribes in the United States, tribal enrollment can be determined by lineal descent, as opposed to a minimum blood quantum. Lineal descent means that anyone directly descended from original tribal enrollees could be eligible for tribal enrollment, regardless of how much native blood they have.
-- Wikipedia