75 (seventy-five) is the natural number following 74 and preceding 76.
In mathematics

75 is:

the fourth ordered Bell number, and counts the number of weak orderings on a set of four items.
the sum of the first five pentagonal numbers, and therefore a pentagonal pyramidal number.
an enneagonal number.
a Keith number, because it recurs in a Fibonacci-like sequence started from its base 10 digits: 7, 5, 12, 17, 29, 46, 75…
a self number because there is no integer that added up to its own digits adds up to 75.

Excluding the infinite sets, there are 75 uniform polyhedra (or 76 if edges are allowed to coincide).
In other fields

Seventy-five is:

The atomic number of rhenium
The age limit for Canadian senators
A common name for the Canon de 75 modèle 1897, a French World War 1 gun
The departement number of the city of Paris
Municipal Okrug 75, a municipal okrug in Frunzensky District of the federal city of St. Petersburg, Russia
The number of balls in a standard game of Bingo in the United States
In Bible is the number of people that left Canaan to go to Egypt and meet with Joseph. “Acts 7:14”