What is Lineal?

Legal Definition
That which comes in a line. Lineal consanguinity is that which subsists between persons, one of whom is descended in a direct line from the other. Lineal descent, is that which takes place among lineal kindred.
-- Bouviers Law Dictionary
Legal Definition
That which comes in a line; especially a direct line, as from father to son. Collateral relationship is not called "lineal," though the expression "collateral line," is not unusual.
See also
-- Black's Law Dictionary
Legal Definition
In a direct line of descent.
-- Ballentine's Law Dictionary