Signals and systems/Unit step function
The unit step function, also known as the Heaviside function, is defined as such:
![](http://upload.wikimedia.org/wikipedia/commons/e/e0/Unit_Step_function_u%28t%29_-_SST_-_08DEC05.png)
![](http://upload.wikimedia.org/wikipedia/commons/6/67/Unit_Step_function_u%28t-to%29_-_SST_-_08DEC05.png)
Sometimes, u(0) is given the value of (and sometime also 0 or 1). For many applications, it is irrelevant what the value at zero is.
Source: Wikibooks on Signals and Systems