Habit
A habit is a behaviour that is repeated with some frequency, often in response to some stimulus. Many habits include some amount of automaticity meaning that you might do it without really thinking about it like opening the fridge and looking in every time you enter the kitchen. Some habits are undesirable like eating or smoking in response to stress while others are preferred like brushing one's teeth.
Habits are built by repeated execution of the behaviour with rewards.
Tips for building habits
Have a clear and reliable trigger for your habit. If you want to put a habit on a trigger, the trigger should be sufficiently obvious that you are going to notice it. If you don't notice the trigger you'll be unable to perform the behaviour and reinforce the desired habit. Additionally, if you don't implement the habit every time you notice the trigger, you will be training yourself to not perform the behaviour in response to it.
You can use the completion of one behaviour as the trigger for the next. This is called habit stacking and can be used to build up quite complex routines.