Historically, there are two different meanings for the term "Modern Dance." Originally, the term came
from the early 20th Century meaning that ... the dances (Ballroom type) being done were modern in nature, rather than the older
Step, Minuet etc. of the time. These "Modern Dances" were generally
associated with Vernon and Irene Castle, Maurice
Mouvet and Florence Walton and others who were ballroom dancers of the time (1910s.)
Today the term means something different however. This term 'Modern Dance' designates a concept of dance rather than a particular dance School or dance Studio. By the 1920's it was done by a solo or group - a free form type dance with no base in its beginnings but utilized many other styles of dance movement (originally Greek and Oriental,) as well as indigenous and the dancers personal feelings leading to movement.
Isadora Duncan (1887-1927) was to become the mother of (current day) freeform style called 'Modern Dance' and by 1913 had reached international fame with others soon following suit.
This new free form style was thought ridiculous at first but Duncan's success with this style in America opened many eyes along with other pioneers such as Ruth St. Denis (1878-1968), Rudolf Van Laban and Mary Wigman who would become the primary movers of this style of dance and helped create a new style of dancing. Others to follow and adding more to this ever growing style were such dancers as Hanya Holm (1893-1992),
Ted Shawn (1891-1972), Doris Humphrey, Charles Weidman (1901-1975), Martha Graham (1894-1991), Valerie Bettis (1919-1982), Jose Limon, Helen Tamiris (1905-1966), and Tórtola Valencia (1908-1930) with her 'Natural Dance' (in no particular order.)
The Modern dance form came to be due to the restrictions on the ballet dancers at the time or to "release the dancer from explicit and locked definition" but today modern dance is very recognizable thru its forms, base and techniques. The dancers use their natural instincts, impulses, a release of natural feeling when moving or a "firstness." The dancer tries to release the inner feelings in an outer expression. It works some what of the 'Ying and Yang' and or 'Contrast' teachings of opposites, such as Up/Down, Left /Right, Contraction/Release, Fall/Recover, Fast/Slow etc. The body is the instrument for self indulged expression.
Modern dance avoids any set bases or a set vocabulary of steps such as ballet or Oriental, Hindu etc. but does use all these ideas along with the dancers natural ability and prior dance training (Its kinda like a free for all or 'on the spot type choreography' or the "Im'a Tree" idea.) Because of this, unfortunately, Modern dance is often misunderstood by the onlooker, as occasionally they can not figure out what is happening. By the 1950's, Modern dance was firmly established in the dance world.
Modern dance is said to be based on four principles: 1) Substance (movement) 2) Dynamism, 3) Metakinesis, 4) Form (not dependent on the music.)