An assumption is defined as something taken for granted. That is, it refers to something an individual takes to be true. When a person makes an assumption, he acts, without verification, as if something is true.
The word assumption is synonymous with the word guess, but it carries different connotations. Using the word guess generally implies that a person is aware of a statement's uncertainty. Meanwhile, people make assumptions either consciously or unconsciously. For example, the sentence "Jared was late to work today, because he assumed there would be little traffic on his commute" implies that Jared made an oversight by not thinking about all the possible reasons for traffic. Meanwhile, the sentence "Jared was late to work today, because he guessed there would be little traffic on his commute" implies that Jared did think about the possibility of traffic.