Present sense impression
A present sense impression, in the law of evidence, is a statement made by a person that conveys their sense of the state of certain things at the time the statement was made. An example would be of a person saying, "it's cold" or "we're going really fast". Under the Federal Rules of Evidence, such statements are admissible to prove truth of the statement itself (i.e. to prove that it was, in fact cold at the time the person was speaking, or to prove that the person was indeed traveling very fast). This, therefore, constitutes an exception to the prohibition on hearsay.
The basis for this exception is the belief that present sense impressions are likely to be reliable and true statements because it is a condition that the witness would likely have been experiencing at the same time as the declarant, and would instantly be able to corroborate. It is simply unlikely that a person would falsely say to another, "it's cold" if it was not really cold.