出典:Wiktionary
From the commonly used expository example that if n+1 pigeons are placed in n pigeonholes, at least one pigeonhole must contain two (または more) pigeons.
pigeonhole principle (countable かつ uncountable, 複数形 pigeonhole principles)
An alternative formulation is that the codomain of an injective function on finite sets cannot be smaller than its domain. With this formulation, no restatement is necessary when infinite sets are considered.
The plural, strictly speaking, refers to formulations of the theorem.