First and foremost, lightweight is a comparison. Is your WM lighter on cpu and RAM than X% of other WMs? Faster? Then it's lightweight.
Yet it's inherently a fuzzy term, as it focuses on user experience, with varying old hardware, bad drivers, and so on. What percentage does one pick, and what metric? (count alone, market share, installed base...)
There may be no clear, well-defined term, but everyone agrees that KDE is heavy