Signal and Space/Time I have been reading a couple of definitions on signal such as:


  Any quantity measurable through time over space or any higher
  dimension can be taken as a signal.   


And this,


  In the physical world, any quantity exhibiting variation in time or
  variation in space (such as an image) is potentially a signal that
  might provide information on the status of a physical system


Taking human voice as example,

A sound signal is converted to an electrical signal by a microphone, generating a voltage signal as an analog of the sound signal, making the sound signal available for further signal processing. Sound signals can be sampled at a discrete set of time points.



I can understand where times comes in but where's the "space" come here according to the definition of signal?
My understanding of your question is that you have a small misunderstanding here. In the definition it is not said time and space. A signal can vary in time or space. Some signals vary with time, as your example human voice that varies over time in air pressure (or equivalently voltage), some vary with space, like image and some vary with both time and space such as video. I feel,  your misunderstanding in human voice example is that you believe there should be an space dimension too in the game.