Introduce a new function in a package?

R-help,

Sometimes I define functions I wish to have in any R session.
The obvious thing to do is copy-paste the code 
The thing is that sometimes I don't know where I have the function
code.

My question is if somehow I could define a function and "introduce" it
(let's say 'base' package ) so that 
could be used anytime I run a different R project.

Thank you in advance