# What is causal signal?

I'm using Digital Signal Processing Principles, Algorithm and Applications 4th Edition written by Proakis and Manolakis. In Chapter 3, subtopic 3.3.2 it mentions the term "causal signal" which I can't find any definition. I know what a causal system means: a system that output depend entirely on past and present input, not future input but here it's causal signal, not system

So what does causal signal definition?

I tried to look at the Index and it says that the term "Causal Signal" is mentioned in page 85 but in fact, I find nothing in page 85 mentioning it

A system is causal if its output depends only on the current input and past inputs (and not on future inputs). As a consequence, if the system is a linear, time-invariant (LTI) system (which input/output relationship can be completely characterized by an impulse response), that impulse response, $h(t)$ must be zero for all time $t \lt 0$.
Some people define a causal signal, $x(t)$, to be one that can be the impulse response of a causal system: it is zero for all time $t \lt 0$.
• Namely, that the signal $x[n]$ is equal to zero for all $n < 0$. – Jason R Nov 18 '15 at 3:43