A lie is an assertion that is believed to be false, typically used with the purpose of deceiving or misleading someone. The practice of communicating lies is called lying. A person who communicates a lie may be termed a liar. Lies can be interpreted as deliberately false statements or misleading statements. Lies may also serve a variety of instrumental, interpersonal, or psychological functions for the individuals who use them.

Generally, the term "lie" carries a negative connotation, and depending on the context a person who communicates a lie may be subject to social, legal, religious, or criminal sanctions; for instance, perjury, or the act of lying under oath, can result in criminal and civil charges being pressed against the perjurer.

Although people in many cultures believe that deception can be detected by observing nonverbal behaviors (e.g. not making eye contact, fidgeting, stuttering) research indicates that people overestimate both the significance of such cues and their ability to make accurate judgements about deception. More generally, people's ability to make true judgments is affected by biases towards accepting incoming information and interpreting feelings as evidence of truth. People do not always check incoming assertions against their memory.