Definition:Comparable Sets/Cardinality

Definition
Let $S$ and $T$ be sets.

Then $S$ and $T$ are comparable (in size) either:
 * $S$ can be put into one-to-one correspondence with a subset of $T$

or:
 * $T$ can be put into one-to-one correspondence with a subset of $S$

or both.

That is, if either $S$ is smaller than $T$ or $T$ is smaller than $S$.