What is range?
From the lowest value to the highest value.
I suppose you're talking about the terms that apply to the concept of function.
The most common definition for this term is the set of values for the dependent variable. That is, it is the set of reals f(x) for all x in the domain. In terms of the graph, the range is the set of y values for all points that satisfy the graph. For example, any line that is not parallel with the y-axis has a range equal to R, the reals. Whereas the sine and cosine functions have a range of [-1,1].
It should be pointed that there is some lattitude in how different authors define this term. Some choose to use the term image for the definition I gave and use range to mean something else. Others ignore the concept of image and define range as I did above. When one looks at different texts it necessary to be aware of how the terms are being used. The word 'range' is one of those terms with shades of differences.
I see DrBob222 replied while I was working out a response.
Yes, range could also be applied to a set of data meaning the difference between the highest and lowest values. This would be a definition for the term in statistics. I wasn't sure what area of study you were intending.
subtract the lower from the higher. higher-lower