What is the difference between “ an indicator” and “ an indication “?
For example, which one should I use in this sentence ?

Dreams are an indicator / indication of what we long for.

Thanks for your time!

Question

What is the difference between “ an indicator” and “ an indication “? 
For example, which one should I use in this sentence ?

Dreams are an indicator / indication of what we long for.

Thanks for your time!

Answer

Both work in the "Dreams..." sentence. As Gracious says, an "indicator" can also be a concrete thing, eg.: Litmus paper is an acid-base indicator.

Answer

An indication is a sign of something whereas an indicator can mean a device that's used to measure stuff.