• [countable, uncountable] a special activity or purpose of a person or thing

  to fulfil/perform a function

  bodily functions (= for example eating, sex, using the toilet)

  The function of the heart is to pump blood through the body.

  This design aims for harmony of form and function.

• [countable] a social event or official ceremony

  The hall provided a venue for weddings and other functions.

  There are numerous functions in aid of charity.

• [countable] (mathematics) a quantity whose value depends on the varying values of others. In the statement 2x=y, y is a function of x. (figurative)

  Salary is a function of age and experience.

Sources: http://www.oxfordlearnersdictionaries.com/