A principle that basically states that things of similar composition will eventually tend toward the same location because they are similarly affected by wind/ocean currents, magnetic fields, and other forces.
Examples are hairballs from dogs/cats and giant masses of condoms people have found in the ocean.
Brad: "...so that was like the third time this year that my sister has been hit in the face with bird shit! I wonder why that happens to her so much."
Jeremy: "Dude, haven't you ever heard of like aggregation? Dee dee dee!"