A continuous-time signal is defined over a continuous range of time values, and its amplitude can take on any value within a continuous range. The continuous-time signal $x(t)$ is a function of a real variable ($\mathbb{R}$): $ \forall t \in \mathbb{R}, x(t) = \ldots $ in the form of a [[mathematical function]] that maps a domain onto a target set: $ x: \mathbb{R} \rightarrow \mathbb{R} $ These signals are often referred to by the abbreviated form of _continuous signals_. A [[continuous-time pure tone]] is an example of a one-dimensional continuous-time signal because the domain only has one dimension. An image is an example of a two-dimensional signal. A signal that is a function of an integer variable ($\mathbb{Z}$) is a [[discrete-time signal]].