Ten (10) is a positive integer one more than 9 and one less than 11.
10 is the base of our number system, called decimal, which is caused by the fact humans have ten fingers.
10 is a composite number, more specifically a semiprime number. It is equal to 2*5.
10 is the fourth triangular number (between 6 and 15). It is also the third tetrahedral number (between 4 and 20). It is the first number besides 1 with both of these properties.
The powers of ten are encountered much more often than other numbers. The next power of ten is 100. In addition, these are the basis of scientific notation.
A number is divisible by 10 if it ends in 0. This makes ten the easiest number to test for divisibility, and is a consequence of ten as our number base.