Exactly what you'd expect -- a number! This data type encompasses integers as well as other real numbers with decimals.
Fun fact: in other lower-level programming languages, like C and Java, numbers are split into integers and non-integers!
We use the number keyword to tell our program the type of variable we're creating. Or, we can let the computer assume based on our variable initialization.
let levelOfFun = 10; let levelOfLameness: number = 10; // This is also fine!