# Translations:Analog linear time-invariant systems/1/en

A continuous-time signal f(t) is a (real or complex) function that is defined for every real number t. The number t represents continuous time. A continuous-time signal also is called an analog signal. An important type of analog signal is the impulse function ${\displaystyle \delta \left(t\right)}$. This function plays the same role as does the discrete impulse function ${\displaystyle {\delta }_{k}}$, but it is not as easy to define. The impulse function ${\displaystyle \delta \left(t\right)}$, which also is known as the Dirac delta function, can be regarded as a generalized function. A generalized function cannot be defined as an isolated entity but instead must be viewed as the limit of a family of functions. Any family of functions ${\displaystyle {\delta }_{\varepsilon }\left(t\right)}$ with the following properties can be used to define the delta function ${\displaystyle \delta \left(t\right)}$. Here, ${\displaystyle \varepsilon }$ is a parameter that characterizes the functions ${\displaystyle {\delta }_{\varepsilon }\left(t\right)}$. The properties are