(n.) A geometric figure composed of a set of tweets on Twitter whose hashtags link such that the tweets form a geometrical figure, in any number of dimensions.
Related term: hashtagonal (adj.), having the properties of such a structure.
Here are three tweets: 1) My #dog ate an #apple. 2) The #lion growled like a #dog. 3) No #lion has ever eaten an #apple. Tweets 1 and 2 link at dog, 2 and 3 at lion, and 1 and 3 at apple. The three tweets form a hashtagonal triangle (or a triangular hashtagon). The same can be done for all geometrical figures reducible to lines and vertices with any degree of connectedness, in any number of dimensions, for flat space and probably for any uniform hyperspheroidal manifold.