Mathematica forms of notations
Following Mathematica's general naming convention, function names in StandardForm are just the capitalized versions of their traditional mathematics names. This shows the cosine function in StandardForm.
This shows the cosine function in TraditionalForm.
Additional forms of notations
Mathematica also knows the most popular forms of notations for the cosine function that are used in other programming languages. Here are three examples: CForm, TeXForm, and FortranForm.