Naruto isn't the best place to search for the jap meaning of the term 'genius', to fully understand the concept, you need to look at other places...

first, the word it self "TENSAI", which means heaven something (possiblt wielder\master), the two translations to it are either genius or prodigy, it'll come in handy.

now, let's look over some geniuses at other animes, for example: Fuji from prince of tennis, Mikagami from flame of Recca and Soujiro from rurouni kenshin. the thing they all shared was the amazing ability to overshadow anyone else and accend over all excpetations, and even while they weren't the best in the respctive animes, they had natrual abilities much higher than the main charecter.

Fuji from prince of tennis was able to go head to head with Tezuka, and even the computer guy (Ino? Inu? something like that) couldn't read and dechipher him, as shown in the game against Mizuki (from 5-0 to 5-7, without allowing one ball to fall in).
Mikagami was prasied by Kage houshi to be above Recca, not becuase of his strentgh, but becuase of his mindset, he beat the stick guy and the the tree dude with amazing ease.
Soujiro got to be as strong as the hitokiri, and even faster than him, at his prime, he was second (maybe even better than) only Shishio, and all that without one thought of anger or malice, and in the tender age of his teens.


in the Naruto anime, you can catagorize the geniuses into three groups.
A: Itachi, Kakashi, Orochimaru, Neji, Sasuke and maybe the 4th.
B:Shikamaru.
C:Lee, Naruto, Gai, Jiraya, Tsunade.

the first group is composed of true geniuses, people who'se skills at being ninjas were obvisouly above others and came in almost natrually.
the second group is a person of the english meanning of genius, a person with high intellgence.
the third group aren't geniuses, they are people who worked hard to achieve thier goals, and even though some of them were talented, it didn't show enough.

when people say 'genius' they mostly mean the first group, A.K.A the prodigies, in my opinion, the word prodigy fitts in much better.