A pod is a group of cetaceans.
Most cetaceans are gregarious, meaning they form groups. This is a very simple form of social behaviour found in certain animals which gather together to migrate (which is the case for some whales), to hunt (such as killer whales), etc.
In cetaceans, killer whales are divided into pods to which observers have given letters (there are 16 pods in the northern Pacific Ocean, 3 pods J, K and L in the southern Pacific Ocean, etc.). Within these pods, each individual is also given a name and number. For example, killer whale 39 in pod J is called Mako.
Cetaceans form pods when they gather together © amateur_photo_bore, Flickr, cc by nc nd 2.0