(1) Having no fixed course. (2) Lacking in consistency.
Example:
In the 1993 World Series, the Phillies weren't helped by the erratic performance of their ace relief pitcher, "Wild Thing."
Erratic can refer to literal "wandering." A missile that loses its guidance system may follow an erratic path, and a river with lots of twists and bends is said to have an erratic course. Erratic can also mean "inconsistent" or "irregular." So a stock market that often changes direction is said to be acting erratically. And Wild Thing's problem was erratic control: he could throw strikes but he also threw a lot of wild pitches.