In dealing with Integral you must adopt the following terminology:
ò symbol is called integral symbol. x is the integration variable. y(x) function is called function to integrate. dx is the variable's differenzial. x1 and x2 numbers are called integral's bounds : x1, placed below integral symbol, is called the lower bound ; x2, placed above integral symbol, is called the upper bound. (x1-x2) interval is the integration interval. (Note that "lower" means below and not necessarily x1<x2. This aspect we be better discussed in Properties section).