I am exploring application of lean startup principles to a niche industry that has nothing to do with entrepreneurship. In other words, the principles wouldn't be applied to creating a start up, but it would treat an individual as the product/start up and invite individuals to identify their own value proposition for employers or in situations where they need to demonstrate value. Is it okay to call a product developed for this purpose "lean" as in "lean career development"?
Ries owns 'Lean Startup' trademark. I doubt 'lean' by itself is trademark-able, and isn't claimed by Eric. Someone else may claim 'lean career development,' so might be worth a trademark search.
A combination of words may be trademarked but that does not protect the words individually. "Lean" or "startup." The focus of trademark law is not protecting the creative work of the owner but protecting consumers- preventing the likelihood of confusion with other recognized products on the market.
Whether or not your material would infringe copyright would depend on whether or not you expressed ideas similarly ( not just having similar ideas).
If you have Tm or copyright questions fell free to call me for a consult.
Everyone who has answered is correct so far. The term and principles of "Lean" have existed for a long time. You can also check with the Lean Enterprise Institute (http://www.lean.org/). The term was coined in the late 80's by MIT prof Jim Womack to describe Toyota's process (http://www.lean.org/WhatsLean/)