Definition of ARCH

Schweser defines an ARCH(1) time series as one in which “the variance of the residuals in one period is dependent on the variance of the residuals in a preceding period.” What is the difference between this and serial correlation? I thought serial correlation is where residuals are correlated, and conditional heteroskedasticity is where the variance of residuals is related to the independent variable. This definition of ARCH sounds more like the former.