The best replacement for function is relation.

The best alternatives to function are rated as following:
★★★★★  relation
★★★  method
★★★  procedure
★★  form
★★  equation
★★  stored procedure
★★  functionality
★★  process
★  non function
★  sub
★  interface marked as restricted
★  not a function
★  feature
★  subroutine
★  sub vba
★  purpose
★  not calculator
  linear equation
  procedure oracle
  staff function
  stored procedure in sql
  relation worksheet
  method java
  variable matlab
  excel
  var function
  task
  in excel
  design
  probability mass function
  dysfunction
  capability
  role
  exponential function
  method c
  business process
  polynomial function
  density function
  formula
  method c#

Your alternative for function is not listed? Suggest!
Speaking about function, one probably shouldn't forget about parameters javascript.

Other search terms related to function and their ratings are:
★★★★★ parameters javascript
★★★ gallbladder
★★★ parameters c++
★★★ derivative
★★★ return python
★★★ domain and range
★★★ return c++
★★ parameter in javascript
★★ limit
★★ group by
★ return value
★ parameter
schemabinding
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Google Trends

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function
relation
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