An Octi (also known as an Octopus) is a backup
regulator for scuba diving. It is so named
because the primary
stage regulator (the bit which sits on top of the tank)
looks like an octopus when it has the extra hose for the
backup second stage regulator.
All divers should take an Octi with them, since it
hardly uses any more room, yet gives both you and your
buddy an alternate air source. Trust me, it really sucks
(or rather, doesn't), when your primary regulator gets
sand all through it and clogs up.