Meat is defined as the flesh of an animal, and fish are classified as animals. However, people do not always think of fish as a kind of meat. This idea may come from Catholicism, which asks its believers to abstain from eating meat during Lent but allows eating fish.
The basis for eating fish during Lent comes from the writing of the theologian Saint Thomas Aquinas, who argued that fasting should only pertain to those animals that breathe air and walk on dry land. This divide, in turn, has its origins in Judaism. Without giving a reason why, Judaism distinguishes between animals of the land, whose flesh may not be consumed with dairy products under Jewish law, and fish, whose flesh is considered to be in a separate category and is not subject to the same prohibitions against being eaten with dairy products.