What is the abbreviation for word custom?

There isn't a widely recognized abbreviation for the word "custom".

However, you can use these options depending on the context:

* Cust. (especially for orders, invoices, etc.)

* Cstm. (less common, but still understandable)

* Customized (if you want to use the full adjective)

It's always best to be clear and use the full word unless there's a specific reason to abbreviate.