Sardine is a name of several kinds of fish, especially oily fish. Another name for these fish is Pilchard. All of those fish are quite small and are related to the herring. The Latin name of the family of these fish is Clupeidae. However, the naming is not precise. Fishbase, a database about all kinds of fish, has six species just named pilchard. There are over a dozen with the name of sardine. Many more exist, with the basic names qualified by adjectives.
A closed can (tin) of sardines
After opening, the tin looks like this.
Sardines on a plate
Sardines next to a ruler, showing their size