What is stearidonic acid? Is it an omega-3 fatty acid?

Yes, stearidonic acid (STA) is a member of the omega-3 fatty acid family. It has 18-carbons in chain-length (as does alpha-linolenic acid, ALA), but has 4 double bonds (unsaturation sites) whereas ALA has 3. Although fairly rare in occurrence, STA is found in certain oils such as those derived from the seeds of the Echium plantagineum . It gives rise to significantly more EPA in the human body by metabolism (desaturation and elongation reactions) than does an equivalent amount of dietary ALA when either are consumed by humans.