olemus
funktsioon, mille loob programmi või keskkonna kasutaja, erinevalt sisseehitatuist
= a function created by the user of a program or environment, as opposed to a built-in function

ülevaateid
https://www.techopedia.com/definition/3888/user-defined-function-udf
https://en.wikipedia.org/wiki/User-defined_function

näiteid
https://www.educba.com/c-plus-plus-user-defined-function/
https://simplesqltutorials.com/sql-server-user-defined-functions/
https://www.w3resource.com/python/python-user-defined-functions.php
https://docs.snowflake.com/en/developer-guide/udf/java/udf-java-introduction.html