"I am unsure the definition of 'good' in Buddhism."
In general, "good" is that which leads to the welfare of beings, leads to nibbāna. The precepts and the eightfold path present examples of "good". The hindrances, poisons, fetters, etc. are examples of "bad".
"Now assuming, you are surrounded by 'good' people, is that not a form of aversion?"
It is aversion if aversion is present in the mind and/or it's the main driving force producing the influence. By aversion, it is meant a specific unwholesome state. An aversion born of conceit, hate, delusion, ...
Not all "aversions" (in the conventional sense) are aversions (in the buddhist sense). Otherwise, all kinds of avoidances would be unwholesome. Like avoiding walking towards a pit.
Renunciation (eg. avoiding unwholesome states and circumstances that promote unwholesome states) is a prime concern in Buddhism.
"In Buddhism, if i choose to avoid such people, is that aversion?"
Not necessarily. Your state of mind while you're inclined to avoid certain things or people could be any, including compassion for yourself. Moreover, regardless if one perceives aversion in ones mind or not, it's frequently encouraged to associate with good people, and avoid associating with those whose relationships raise or increase hindrances.
I have heard that on one occasion the Blessed One was living among the Sakyans. Now there is a Sakyan town named Sakkara. There Ven. Ananda went to the Blessed One and, on arrival, having bowed down to the Blessed One, sat to one side. As he was sitting there, Ven. Ananda said to the Blessed One, "This is half of the holy life, lord: admirable friendship, admirable companionship, admirable camaraderie."
"Don't say that, Ananda. Don't say that. Admirable friendship, admirable companionship, admirable camaraderie is actually the whole of the holy life. When a monk has admirable people as friends, companions, & comrades, he can be expected to develop & pursue the noble eightfold path.
-- SN 45.2