member( X, [X|Tail] ). member( X, [Head|Tail] ) :- member( X, Tail ).
Read this as:
X is a member of any list whose first element is X. X is a member of any list if X is a member of the tail of the list.
Standard terminology in list processing: The head of a list is its first element. The tail of a list is the sublist formed by its second, third, ... elements.