MKV extension is abbreviated from Matroska Multimedia Container. Matroska is an English word derived from the Russian one - Matryoshka, which stands for a nesting doll (the Russian doll within a doll, aka a babushka doll). This is a play on the container type of the matryoshka doll, i.e. media within a form of media compared to doll within a doll.
MKV is an open standard free Container format that can hold an unlimited number of video, audio, picture or subtitle tracks inside one file. It is intended to serve as an all-purpose format to store common multimedia content, such as movies or TV shows. There are two file types of Matroska container: MKV extension is for video and audio files and MKA extension is for audio-only files.