An improper integral is a type of integral that involves either an infinite limit of integration or an integrand that becomes infinite within the interval of integration. These integrals are used to calculate areas and volumes when traditional methods fail due to unbounded behavior. To evaluate an improper integral, one typically takes a limit. For example, if the upper limit is infinite, the integral is expressed as the limit of a definite integral as the upper limit approaches infinity. This process helps determine whether the integral converges to a finite value or diverges to infinity.