Hivemind is when two or more people come to the same thought at the same time because of the same circumstances but do not know each other beforehand (i.e. it cannot be an inside joke). This usually happens on internet message boards, and is especially noticeable when the thought reached is not a meme. The name is reflective of insects who act in unison with their hive (or nest) mates.
Poster 1: *Random picture*
Poster 2: Holy shit, that looks just like my mom!
Poster 3: omg, that looks just like my mom
Poster 4: I'd like to meet your mom. Also, hivemind.