How should I select elastic for a sewing project? I want to adapt a pair of pants to add elastic to the waistband. I have a particular "stretchiness" in mind for this project, but don't know how that relates to a given thickness, width, material, or other characteristic of any particular elastic product.

Is there any standardized terminology for relative stretchiness that I could use when talking to a sales assistant? Are there better ways to choose what elastic to use for a project than standing in the store and pulling on samples to guess how well they would work?

I'm looking for general advice rather than "here's what's best for your pants" -- because this does come up in multiple projects :)
This is commonly called the "stretch percentage" or "stretch factor". The name comes from how much percent of the original width/length the material can stretch. For example, if you can stretch a 4" piece of elastic to 6" long, the stretch percentage would be 50%.

Here's a simple visualization from Patterns for Pirates:



As a side note, some patterns come with a "stretch gauge", instead of telling you a stretch percentage. It's usually printed on the back of the envelope so you can easily measure your fabric against it:



What elastic to use really depends on your particular application! A thicker or wider elastic may be more sturdy but less comfortable, for instance. As a general rule, thicker elastics seem to be less stretchy - but it's a very general rule. If you know the measurements you want for the finished pants, you can come up with the required stretch percentage: (fabric width / fitted width - 1) and bring that gauge to the store to find which ones are suitably stretchy.