Term for minimum or maximum

I need to ask a user to request either a minimum or maximum value. What would be the appropriate label for this? I have considered extremum but I am not sure if this is commonly understood or appropriate.

Example: Is the user interested in a companies lowest or highest stock price over the last 6 months? They will have the option of choosing 'min' or 'max'.

Mathematicians call this either an extremum (plural extrema) or extreme value. But as you say, this is unlikely to be understood by non-technical users. I doubt that there is a better term for it, however, since mathematicians would probably not have come up with a special word for this if there was an ordinary English word or short phrase that meant the same thing.

I would recommend you just use "maximum or minimum value".

You could refer to the bounds of the range. From NOAD:

bound 2
noun (often bounds)
a territorial limit; a boundary : the ancient bounds of the forest.
• a limitation or restriction on feeling or action : it is not beyond the bounds of possibility that the issue could arise again | enthusiasm to join the union knew no bounds.
• technical a limiting value.

The "limiting" value can refer to the upper or lower limit.