In Greek Canon means rule, the sample. In life we often come across this word. We say, "On strict rules...". Or use the notion of canonical in the sense of a model, obeying the strict rules. In music the meaning of this term much already. The so-called polyphonic piece, in which all voices sing the same melody, but not together, and engaging alternately, sometimes from the same sound, sometimes from different. Actually, the Canon is a musical piece or the episode built on the ongoing simulation. An example of this episode is the duet between Onegin and Lensky in a duel scene in Tchaikovsky's "Eugene Onegin". More recently, inseparable friends, they are one against the other with pistols in their hands. Their thoughts are the same ("Enemies! How long have each other we thirst for blood was taken?"), therefore, the melody is the same; but they are not together, they are enemies - and therefore their melodies do not coincide in time, each sound by itself. So subtle, psychologically reasonably used the old composer polyphonic form.
P. I. Tchaikovsky. The Duo Lensky and Onegin "Enemies" from the Opera "Eugene Onegin"
A special kind of Canon - infinite Canon. The melody can be repeated any number of times, moving from the end back to the beginning.