**Diagonal intersection** is a term used in mathematics, especially in set theory.

If is an ordinal number and is a sequence of subsets of, then the *diagonal intersection*, denoted by

is defined to be

That is, an ordinal is in the diagonal intersection iff it is contained in the first members of the sequence. This is the same as

where the closed interval from 0 to is used to avoid restricting the range of the intersection.

### Famous quotes containing the word intersection:

“You can always tell a Midwestern couple in Europe because they will be standing in the middle of a busy *intersection* looking at a wind-blown map and arguing over which way is west. European cities, with their wandering streets and undisciplined alleys, drive Midwesterners practically insane.”

—Bill Bryson (b. 1951)