The definition I know can be found in [Lectures on motivic cohomology][1]. It is definition 1A.2.


  [1]: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwjFhvjXl6-BAxUWQ_EDHSwRCEIQFnoECCcQAQ&url=https%3A%2F%2Fwww.claymath.org%2Flibrary%2Fmonographs%2Fcmim02.pdf&usg=AOvVaw2MdXGFGg-wNk8qUVzE5_Jb&opi=89978449