What is causal signal?
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 $t \lt 0$.