The fundamental result in LTI system theory is that any LTI system can be characterized entirely by a single function called the system's impulse response. The output of the system y(t) is simply the convolution of the input to the system x(t) with the system's impulse response h(t). This is called a continuous time system. Similarly, a discrete-time linear time-invariant (or, more generally, "shift-invariant") system is defined as one operating in discrete time: yi = xi * hi where y, x, and h are sequences and the convolution, in discrete time, uses a discrete summation rather than an integral.