From Wikipedia:

A subroutine, (also known as a procedure, function, routine, method, or subprogram) is a portion of code within a larger program that performs a specific task and is relatively independent of the remaining code.

The content of a subroutine is its body, the piece of program code that is executed when the subroutine is called or invoked.

A subroutine may be written so that it expects to obtain one or more data values from the calling program (its parameters or arguments). It may also return a computed value to its caller (its return value), or provide various result values or out(put) parameters. Indeed, a common use of subroutines is to implement mathematical functions, in which the purpose of the subroutine is purely to compute one or more results whose values are entirely determined by the parameters passed to the subroutine. (Examples might include computing the logarithm of a number or the determinant of a matrix.)

However, a subroutine call may also have side effects, such as modifying data structures in the computer’s memory, reading from or writing to a peripheral device, creating a file, halting the program or the machine, or even delaying the program’s execution for a specified time. A subprogram with side effects may return different results each time it is called, even if it is called with the same arguments. An example is a random number function, available in many languages, that returns a different random-looking number each time it is called. The widespread use of subroutines with side effects is a characteristic of imperative programming languages.

A subroutine can be coded so that it may call itself recursively, at one or more places, in order to perform its task. This technique allows direct implementation of functions defined by mathematical induction and recursive divide and conquer algorithms.

A subroutine whose purpose is to compute a single boolean-valued function (that is, to answer a yes/no question) is called a predicate. In logic programming languages, often[vague] all subroutines are called “predicates”, since they primarily[vague] determine success or failure.[citation needed] For example,any type of function is a subroutine but not main()

It is a common unit of code for most other programming languages.

Function also has a mathematical definition, which is important in computer science.

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