Attempt of a Definition

In Contra Dancing, a group of people dances devised interwoven geometrical movements in congruence with the structure of the music.

... a group of people ... : Most contras are danced in lanes, where one line of dancers faces another line. But there are many other forms, which I also count among the contra dances, if the other criterias are fulfilled.

... devised movements ... : Movements in contra dancing must be devised beforehand, to fit the structure of the music. In most cases they are taught and walked through, before they are danced with music.

... interwoven geometrical movements ... : Round Dance and Line Dance too consist of devised movements. But these movements are danced by single dancers or single couples, and are not related to a group.
Whether a sequence of circles, stars and swings may be considered as "interwoven" is debatable. Perhaps "interlinked" would be a better term?

... in congruence with the structure of the music : Contra dance music must have a distinct structure. However, it must not always be A-A-B-B.
Somewhere I read that music is sound with a structure and a purpose.

Where can I dance?
Contra Dance Links (quite a listing)
Basic Rules for Contra Dancing
Different Styles of Contra Dancing
Formationes in Contra Dancing
How You Learn To Call Contras
Contra Dances written by Heiner Fischle amended 2010-10-29
A Guide to Contra Dance List of Contents
Beat versus Phrase by Hal Rice, Author of "Western Style Contra"
History of Square and Contra Dancing
The earliest Contra Dance
E.O.N.S. the perfect start for a One Night Party
Frequently Asked Questions about Open Country
Figure Eight How to dance it
Hey for Four (Reel of Four) How to dance it
Hey for Three (Reel of Three) How to dance it
Turn the Contra-Corners How to dance it
Short description of specific movements in Contra Dancing (PDF-file, 371 KB, bilingual)
The term “Chain” 300 years ago
Polka Dot A 5 Person Dance
Different meaning of terms between Square and Contra Dancing
Contra Dance CDs I own with list of contents and an evaluation

  Seminar Reports (mostly short dance descriptions)