Real Dictionary

custom
If a shop has your custom, you regularly buy things there.
If it is your custom to do something, you usually do it in particular circumstances.
If you use custom to describe something such as a vehicle or a piece of clothing, you mean that it has been designed for one particular customer.
variable noun: A custom is an activity, a way of behaving, or an event which is usual or traditional in a particular society or in particular circumstances.

custom in American English
a usual practice or habitual way of behaving; habit
made or done to order or, sometimes, made extra fine, as if to order
adjective: made specially for individual customers
noun: a habitual practice; the usual way of acting in given circumstances

custom in British English
adjective: made to the specifications of an individual customer (often in the combinations custom-built, custom-made)
noun: a usual or habitual practice; typical mode of behaviour

custom in Retail 1
noun: If you give a store or business your custom, you buy from them.

custom in Retail 2
noun: A store or business's custom is all its customers.