Eicosapentaenoic acid (or EPA) is a long-chain (LC) poly-unsaturated fatty acid (PUFA). Together with ALA and DHA it is one of the main omega-3 fatty acids.
The long-chain designation stems from the fact that EPA consists of a chain of 20 carbon atoms. Poly-unsaturated fatty acids are characterized by the fact that they have double (=unsaturated) bonds between carbon atoms in multiple ("poly" in Latin) locations in the fatty acid chain. EPA has five such double bonds.
The term "omega-3" refers to the location of the first double carbon bond in the fatty acid chain. Given the fact that omega is the last letter in the greek alphabet and the fact that the first double bond between carbon atoms in EPA is located 3 carbon atoms away from the tail (=omega) end of the chain, it is called an omega-3. Sometimes omega-3 is referred to as n-3 and since EPA has 20 carbon atoms, 5 double bonds and is an omega-3 fatty acid, its shorthand-code is C20:5n-3.