Abverbs of doubt
Adverbs of doubt are those that indicate insecurity or indecisiveness about the action of the verb in the sentence. They modify the verb giving this type of information.
Some common adverbs of doubt are:
A lo mejor me apunto al club de natación.
(I might join the swimming club.)
Quizás compre aquel cuadro que vimos ayer.
(I may buy that picture we saw yesterday.)
Tal vez me acepten en esa universidad.
(They might accept me at that university.)
Note that when they are formed by a group of words they are called "locuciones adverbiales", i.e. "a lo mejor", "tal vez".