According to the Merriam-Webster Collegiate Dictionary:
1 : not proud or haughty : not arrogant or assertive
2 : reflecting, expressing, or offered in a spirit of deference or
3 a : ranking low in a hierarchy or scale : INSIGNIFICANT,
UNPRETENTIOUS b : not costly or luxurious
I think the above is OK. In my mind, I think one definition of being humble is accepting that what I think may not be the correct answer and that someone else's view may be better or at least worth considering. And being gentle in my actions and spirit.
For me, a definition of being humble can be difficult to completely capture with words, and sometimes I define it by thinking of some of the people who have touched me in my life who I have admired for among other things, their humility. Like my late aikido sensei, one of my best friends, my favorite college professor, an ex-girlfriend, etc. Some traits that all of these people have or had were: intelligence, caring, giving, gentle, quiet (until you got to know them), talented in some way, and passionate, among others. Regarding these individuals' intelligence, talents and passions, you never really understood or saw these until you knew them for awhile. Seeing as I regard humility as a very worthwhile trait, I view these and other people as role models in many ways, and on the other side, I also look to other people who exhibit the opposite definition of humility as how not to behave. Anyway, my two yen's worth.