From the Latin word "modulatio", which literally means compliance measures, regularity, occurred Italian modulazione, in translation - play voice. In music theory, the word "modulation" means the transition from one tonality to another. Modulation can be smooth, gradual, there may be sudden and unexpected.
If the new key is not fixed for a long time, and going back to the original tonality, this modulation is called deviation.
Modulation is very important, bright and colorful expressive means of music.