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1.
A person with whom you form a special bond because of the common interest for a specific activity.

A person with whom you get ritually together to perform a specific activity.
I’m not looking for a friend, just an activity partner.
by Paul Ross September 29, 2006
 
2.
Someone to do activities with like tennis, biking, jogging, etc. An activity partner can also be synonymous with dating.
I met this woman on a dating site and we play tennis regularly. We are activity partners.
by noname2010 June 12, 2010