1. a noteworthy or an extraordinary act or achievement usually displaying boldness, skill, etc.
2. Obsolete*. a specialization, or skill.
* Actually, in Dungeons and Dragons we still use the term feat to refer to a specialization or skill.
EX. With a total of about four appearances in Action Comics, one appearance in Superman #361, and a sixth appearance in Green Arrow, Captain Strong is a super hero parody of Popeye the Sailor Man; captain of The Fantasia, he performed amazing feats of strength by consuming a plant with incredible properties called "sauncha" that also had the side-effect of making him act irrational, dangerously aggressive, and suffer from withdrawal symptoms.
With Superman's help, he went to rehab, quit using "sauncha", and swore to only eat vegetables to build up his already remarkable strength. Ever since he has continued to do good where he can, befriend both Clark Kent and Superman (unaware of the fact they're the same person), and serve as both an analogue for drug abuse and for proper nutrition.
And perhaps that is the greatest feat of all.