Definition
Omega-3 fatty acids are a type of unsaturated fatty acid that helps build the membranes of your cells. Their signature feature is that the first double bond shows up at the third carbon, counting from the tail end of the carbon chain. Your body can't make enough of them on its own, so you have to get them from food.
Why is there a "3" in the name?
A fatty acid molecule is a long chain of carbon atoms, like train cars hooked together. One end has an acidic group (called a carboxyl group). Think of that as the front of the train. The very last carbon at the opposite end is called the "omega (ω)" carbon, named after the last letter of the Greek alphabet.
Now count forward from that last tail car. At the third carbon, you hit the first double bond. That's where the name omega-3 fatty acid comes from: a double bond sits at the third carbon from the omega end.
In natural omega-3s, this double bond is the cis type. It makes the carbon chain bend at an angle instead of staying straight. Because of that kink, the molecules can't pack tightly together. So even in cold temperatures, the oil doesn't freeze solid. It stays liquid and flows smoothly.
The secret to staying fluid in cold seas
Picture a fish swimming in the freezing Arctic Ocean. If its cell membranes were made only of saturated fat, like lard, they would stiffen in the cold water. Its cells would struggle to do their jobs.
But the bodies of oily fish like salmon and mackerel are rich in unsaturated fatty acids with bent chains. Those bent tails create little gaps between the molecules in the membrane. Even in biting cold, the membrane keeps its soft, flexible fluidity.
Living cells need flexible membranes to stay alive. A soft membrane lets a cell receive outside signals quickly and swap materials smoothly. This matters most in brain nerve cells and the retina, which constantly send electrical signals. That's why they are packed with DHA, an omega-3 molecule.
A bit more precisely: making it vs. eating it
To be more precise, humans and other mammals can make most fatty acids inside the body. But we lack the enzyme that puts a double bond at the third carbon from the omega end. So alpha-linolenic acid, the starting material for omega-3s, has to come from food. That makes it an essential fatty acid.
There isn't just one omega-3. Alpha-linolenic acid (ALA) is abundant in plant seeds. EPA and DHA are abundant in marine life. Your body converts some ALA into EPA and DHA, but the conversion rate is low. The three also differ in chain length and number of double bonds, so each plays a slightly different role in the body.
Supplements come in several forms: triglyceride (TG), which is the natural form in fish oil; ethyl ester (EE), made by reacting the oil with ethanol; and rTG, where EE is converted back into triglyceride. Some studies say absorption differs by form, but the results conflict, so it's hard to say any one form is better. There are also reports that absorption changes with conditions, such as how much fat you eat along with it.
Common misconceptions
Only certain processed forms of omega-3 that absorb well actually work.
Studies disagree about how well each form is absorbed. Absorption also changes with conditions like what you eat alongside it, so you can't say that only one form is absorbed properly.
Fat is bad for you, so the less omega-3 fatty acid in your body, the better.
Fat is a core building block of cell membranes. Omega-3s in particular can't be made from scratch by the human body, so you need to get the starting material, alpha-linolenic acid, from food.
Where you meet it
EPA and DHA, found in abundance in oily fish like mackerel, Pacific saury, and salmon
Alpha-linolenic acid (ALA), found in plant foods like perilla oil, flaxseed oil, and walnuts
In one sentence
Omega-3s are essential unsaturated fatty acids with a double bond at the third carbon from the chain's end. That bond bends the chain and keeps cell membranes flexible.