Colloquialisms in Math Terminology

Question

What are some of your favorite colloquial sounding names for mathematical objects, proofs, and so on? For example, manifolds are often described using an atlas and a neighborhood describes a small set surrounding a point.

Answer 1

I especially like the agricultural theme very consistently upheld in sheaf theory (stalks, germs, ...).

Answer 2

There's the arboreal theme in graph theory (forests, trees, leaves, roots, ...).

Answer 3

I can't believe no one has yet mentioned the following. These were quite common, at least at one time (1970s).

abstract nonsense -- refers to (Eilenberg/MacLane) category theory

invariant under change of notation -- refers to differential geometry, especially when tensors are involved

proof by diagram chasing -- refers to a proof involving commutative diagrams, especially when the proof is essentially solving a path puzzle or hedge maze puzzle