modern dance is whatever you want it to be.

it is movement, time, space, music (or non-music).

modern dance as an idea is the concept of rejecting everything learned and just dancing.

modern dance as a thing is the movement that made this idea acceptable and more well known.

the modern dance movement began in the late 1890s/ early 1900s by Isadora Duncan and Ruth St. Denis. both rebeled agaisnt the rigid structures of ballet, where only certain movements are allowed. Martha Graham, Jose Limon, Paul Taylor, Alvin Ailey, and many others continued this tradition.