User:RandomP/articles I am waiting for

Another outsourced watchlist. This is for articles that currently just don't even properly define what they are talking about:

• accumulated cyclone energy — unit weirdness. Lacks real references on how good a measure of cyclone strength it is, as others appear to be suggesting using the third power of wind speed in the integral.
• activity (chemistry), pH, and activity coefficient fail to point out how activity is normalised: I'm now guessing it's chosen so that activity = concentration in the low concentration limit, but the articles don't say so.
• opportunity cost — as far as I can tell, the term has no accepted definition, and is used only in compound phrases along the lines of "failing to take into account opportunity cost"