When a muscle moves or spasms suddenly, it's a tic. You can also think of a tic as a habitual movement.
If you have an urge to blink your eyes, move your head in a certain way, or clear your throat repeatedly, each of those movements can be described as tics. Some people make unintentional vocal sounds that experts call phonic tics. Some tics are completely involuntary, while others are more of a response to a strong urge. There are many different causes of tics, ranging from genetic disorders to stress.