Definite Integral:An integral evaluated over a specific interval, producing a numerical value corresponding to the area under a curve between two points.
Change of Variable: A method used to simplify an integral by substituting a new variable for an existing one.
$u$-Substitution: $u$-Substitution is a technique for simplifying integrals by substituting part of an integrand with a single variable $u$ and adjusting differential elements accordingly.