There are a large number of places where it could be defined.
For a full explanation, you need to look at the bash manpage, under INVOCATION, but I have to warn you that it will probably give you a headache before you get it all figured out :-)
Depending on how the shell is invoked and what files exist, it may be one or more of:
/etc/profile./etc/bash.bashrc~/.bash_profile.~/.bash_login.~/.profile.~/.bashrc.- whatever is specified with the
--rcfile option.
- whatever is specified with the
BASH_ENV environment variable.
- whatever is specified with the
ENV environment variable.
- any file called from the preceding files.
For what it's worth, I usually make sure I have a ~/.bash_profile (since that's the highest priority of the set {~/.bash_profile, ~/.bash_login, ~/.profile} - it will only process the first that it finds in that group).
I also make sure I have ~/.bashrc and that ~/.bash_profile sources it at the end.
That way, ~/.bash_profile holds only the stuff that I need in login shells and ~/.bashrc holds the stuff I want in every shell. You just have to make sure that the latter doesn't hold any "cumulative" commands (like adding something to the end of the path).
There's other ways to set it up, especially if you're using different shells and have to share profiles among them, but that scheme above is the easiest I've found in bash-only setups.
