From Gary Garrison, National Chair of NPP:       

A Ten-Minute Play is a play with at least two characters that is not a scene, skit or sketch. Structurally, it should have a beginning, middle and end--just like any good one-act or full-length play. Reaching beyond the surface, the text should be enriched with subtext.Since we only have ten minutes to bring the story full circle, a dramatic conflict should be posed as quickly as possible. The resolution of that conflict is what plays out across the remaining pages. The true success of a Ten-Minute Play is reliant on the writer's ability to bring an audience through the same cathartic/entertainment experience that a good one-act or full-length play accomplishes--i.e., sympathetic characters with recognizable needs encompassed within a resolvable dramatic conflict.

While not wanting to oppress anyone's creativity, recognize that a Ten-Minute Play will undoubtedly be presented in an evening of Ten-Minute Plays. Therefore, elaborate settings, multiple characters, extravagant productions values, etc., could conceivably eliminate your play from consideration.

Finally, do your readers a favor: ten minutes means eight or nine pages, but certainly no more than ten pages. READ YOUR PLAY OUT LOUD to see how it times out using standard playwriting format, 12 pt. Times New Roman font.