Prof. Solum's Legal Theory Lexicon here attempts to debunk, rather than define, the terms "strict construction" and "judicial activism", which he does by showing that straightforward attempts to define the terms fail for one reason or another.
He concludes that "'strict construction' and 'judicial activism' are simply not very useful as theory terms for academic constitutional lawyers." He fails, however, to explain why their use is so widespread. The obvious answer seems to be that they are useful terms of propaganda. It's much easier to rail against judicial activists and offer strict constructionist judges as an alternative if you don't have to state your objections or proposed alternatives with any clarity or specificity.