I searched it on yandex images but there doesn't seem to be a really exact definition? Isn't it a noun?
@Arhad said it right. To his answer, I would only add that on some occasions "раз" is used in the meaning "once":
Раз явился я на работу раньше положенного.
In combination with "однажды", it can mean "once upon a time", though this is a rare situation (and very conversational). This is an example from Vyssotsky's song:
Раз однажды гулял по столице я.
Also, in gymnastics or in the army, your coach or sergeant can command "Раз-два!" оr "Раз-два-три!" to give you rhythm.
To add to all the previous explanations, Раз is one of these multifaceted words in Russian, that as a part of speech, раз can be not only a noun - it depends on its use and meaning, and in time and with practice you will get used to the complexity of using it.
As a noun, раз can indicate an instance of previous events, situations, actions, as @Arhad gave examples:
2) "once again he did it" = в который раз он это сделал;
or an occurrence,
3) "once and for all" = раз и навсегда*; What time do I hee him (again)? = Который раз я его вижу?
or indicate how much something has increased/decreased:
Цены на хлеб выросли в пять раз. - Bread prices have increased five times.
as a pronoun, it means "one", like in the above example: "Раз-два!" оr "Раз-два-три!" or in a famous children's song:
Раз словечко, два словечко - будет песенка. - One - word, two - word, here goes a song.
as an adverb, раз means once, one day, once upon a time:
Раз в крещенский вечерок девушки гадали. (с) Vasily Zhukovsky - Once on Epiphany Eve, girls were fortune-telling.
It can also act as a verb, indicating unexpected quick action:
Ньютон сидел под деревом и раз - ему на голову упало яблоко. Newton was sitting under a tree and poof - an apple fell down onto his head.
Раз can also serve as a conjunction:
Раз ты не хочешь делиться яблоком, то я не поделюсь с тобой шоколадкой. Since you don't want to share your apple with me, I won't share my chocolate with you.