900 (nine hundred) is the natural number following eight hundred ninety-nine and preceding nine hundred one. It is the square of 30 and the sum of Euler's totient function for the first 54 integers. In base 10 it is a Harshad number.
Nine hundred is also:
A telephone area code for "premium" phone calls in the North American Numbering Plan
In Greek number symbols Sampi ("Great Pi" ostensibly from the visual resemblance to the Greek letter), which is signified by a convex horizontal arc with two parallel vertical lines below. Sometimes represented in a slightly tilted form.
A skateboarding trick in which the skateboarder spins two and a half times (360 degrees times 2.5 is 900)