Anything related to transfer functions, i.e. those complex-valued functions of a complex variable used to model mathematically the I/O relationship of linear time-invariant initially-at-rest system using Laplace transform.
Anything related to transfer functions, i.e. those complex-valued functions of a complex variable (typically named s, a.k.a. complex frequency) used to model the I/O relationship of linear time-invariant (LTI) initially-at-rest systems using Laplace transform.
Given an LTI analog system whose impulse response is \$h(t)\$, its transfer function is $$ H(s)=L[h(t)] $$ Where L indicates the Laplace transform. It turns out that the zero-state response \$y(t)\$ of the system to the input signal \$x(t)\$ can be easily obtained in the s-domain by the formula: $$ Y(s) = H(s) X(s) $$ where uppercase letters indicate the Laplace transform of the signals involved.
See also Wikipedia on transfer functions.