# 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* . This function plays the same role as does the discrete impulse function , but it is not as easy to define. The impulse function , 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 with the following properties can be used to define the delta function . Here, is a parameter that characterizes the functions . The properties are