Vermutlich, wahrscheinlich, möglicherweise, vielleicht* = presumably, probably, possibly, maybe   Vermutlich hat er es verloren. Presumably he has lost it.

* The difference between those four is quite subtle.

Wahrscheinlich and vermutlich mean that the action seems to be quite probably. Vielleicht and möglicherweise mean that the probability is a bit smaller than with wahrscheinlich and vermutlich.

Compare these phrases   Vielleicht hat er es verloren. Maybe he has lost it.   Wahrscheinlich hat er es verloren. Probably he has lost it.   Vermutlich hat er es verloren. Presumably he has lost it.           Vielleicht weiß er es. Maybe he knows.   Wahrscheinlich weiß er es. Probably he knows.

In the following sentences you'll find the difference more clearly.   Wahrscheinlich weiß er es, aber er will es uns nicht sagen. Probably he knows, but he doesn't want to tell us.   Vielleicht weiß er es, aber er will es uns nicht sagen. Maybe he knows, but he doesn't want to tell us.

In the first sentence there is little doubt that "he knows". In the second sentence the doubt is stronger. However the difference is subtle.