Friends can rely on each other. If you feel upset, lonely depressed, you call them.
Friends are here for you, unless they have something important. They define themselves what is important or not. But if so they come back to you as soon as they are available.
They care about each other. A Friend won't try to cheat you or consciously introduce you to unreliable people, people who may hurt you.
Friends don't kiss or fuck. If these happen you enter a gray zone "the friend with benefits" countdown. At the end of the countdown you re-evaluate the relationship.
If a Friend start having feeling for the other, he/she has to say it.
If a Friend feel that his/her Friend start having feeling for them, they should make it easy for the other person to raise the topic, or raise the topic themselves.
Friends don't drop the ball. When you contact them they always come back to you, it can be in the hour, within a day, a month or after several years. But they do keep sending the ball back at you.
Friends enjoy each other company. No artificial needed, no muss no fuss.
Friends accept their difference and always try to understand and not judge.
Friends are people you admire in some aspect and that admire you in other aspect.
Friends have the same definition of friendship.
If you don't respect one of this rule, you are not Friends according to my definition. However everyone has a different definition, if lucky you will find someone else with the same definition than you.