Real Dictionary

guess
verb: If you guess something, you give an answer or provide an opinion which may not be true because you do not have definite knowledge about the matter concerned.
verb: If you guess that something is the case, you correctly form the opinion that it is the case, although you do not have definite knowledge about it.

guess in American English
to form a judgment or estimate of (something) without actual knowledge or enough facts for certainty; conjecture; surmise
intransitive verb: to form an estimate or conjecture
noun: an opinion that one reaches or to which one commits oneself on the basis of probability alone or in the absence of any evidence whatever
transitive verb: to arrive at or commit oneself to an opinion about (something) without having sufficient evidence to support the opinion fully

guess in British English
to form or express an uncertain estimate or conclusion (about something), based on insufficient information