A suspect is a person who is believed to be guilty of a crime. If you leave the scene of a murder with blood on your hands and a weapon in your pocket, you’re likely to become a prime suspect.
If others believe you have committed a crime, you are a suspect. The word can also be used as a verb and an adjective. To suspect someone of something is to believe that they probably did it. Do you ever suspect your parents of taking some of your Halloween candy? If something such as someone's excuse or motive is suspect, it seems a bit off. That bad-smelling piece of fish is suspect — you'd better not eat it.