Research into music video: 1920 - 1959....

Musical films were also fundamental to the development of music video. Some well-known music videos have imitated the style of classic Hollywood musicals from the 1930s to the 1950s. An example of this is Madonna's 1985 video "Material Girl" was reflective of Jack Cole’s staging of “Diamonds Are A Girl’s Best Friend” from the film Gentlemen Prefer Blondes. 

                                                                                          

Michael Jackson's dance sequences in some of his videos are influenced by classic hollywood musicals. For example, "Thriller" and "Bad" was influenced by the stylised dance fights in the film version of West Side 

Story. 

Research in music video: 1920 - 1959

The phonoscène era was the beginning of sound film. The recording and film were synchronized by a mechanism patented by Léon Gaumont in 1902. The first phonoscènes were presented by Gaumont in 1902 in France. 
It used a chronophone sound recording with a chronograph film shot with actors lip-synching to the sound recording. (Source: Wikipedia) 

Felix Mayol performing. 

In the  Early 1930s some cartoons featured musicians performing their music on camera in live action segments during the cartoons. Animated films by Walt disney such as the Silly Symphonies shorts and especially Fantasia, which featured classical pieces, were constructed around music. The sing along concept is still used today, for example with younger viewers; high school musical.

Silly Symphony Short. 

What is a music video?

"A music video or song video is a short film integrating a song and imagery, produced for promotional or artistic purposes" 

I think this means that a music video is a short film which is used as a marketing device intended to promote sales of the artists' music. Additionally, it could be made for an artistic purpose which has a unique meaning for the artist.