The Question:

Is there a word that designates a 50th anniversary in the same way that sesquicentennial means a 150th?

The Answer:

Semicentennial generally denotes a 50th anniversary (or describes events or objects pertaining to it). A synonymous, though slightly less mellifluous, noun is quinquagenary.

Some other words along these lines are quinquennial (pertaining to a fifth anniversary, or a period of five years), quindecennial (15th aniversary), vigentennial (20th), and quasquicentennial (125th).

-The Fact Monster