If, elseif…is there an “or” function?

This is simply confusing me to death.

Is there not an “or” function in the php code? If I’m using conditional tags, for example:

<?php } elseif (is_author(‘admin’)) { ?>
Blah blah blah

<?php } elseif (is_author(’18’)) { ?>
blahdy blah blahdy blah

What if I want three different pages, a couple of author archives, and three different categories all to produce the same result? Do I have to input that desired result 8 different times?

I feel like it’s right in front of me, but I’m just missing something.

I hope the question is clear; if not, I can cite a few more examples.

Thanks a bunch if you can help.