What is DOMS (delayed onset muscle soreness)?

In one line

DOMS (delayed onset muscle soreness) is muscle pain and stiffness that shows up about a day after exercise your body is not used to.

What is DOMS (delayed onset muscle soreness)?
The soreness that shows up the next day
The day after your first leg workout in a while, your thighs ache when you walk down the stairs. It felt fine during the workout, but the pain arrives about a day later. This is called DOMS. This post explains what DOMS is, why it comes late, and what it gets confused with.
In one line
Muscle aching about a day after exercise
DOMS is muscle aching and stiffness that appears about a day after exercise your body is not used to. It stands for 'Delayed Onset Muscle Soreness': soreness that starts late.
When does it hurt?
Worst on days 1-3, gone within days
DOMS starts a few hours after exercise and is worst on days 1-3. The pain lasts at most about 5 days and then fades. The strength lost in the meantime also comes back, and no lasting damage remains.
After what kind of exercise?
New exercise, or exercise after a break
DOMS tends to follow exercise the body is not used to. Typical cases are doing a new exercise for the first time, or starting again after a break. The harder and longer the exercise, the more likely it is.
Especially when resisting while stretching
Slowly lowering a weight
Movements where a muscle produces force while it lengthens are especially good at causing DOMS. Your front upper-arm muscle works this way when you slowly lower a dumbbell. This is called an 'eccentric contraction', and downhill running has a lot of it too.
Why does it hurt?
Tiny damage inside the muscle, then inflammation
The most widely accepted explanation today goes like this. Unfamiliar force damages tiny structures inside the muscle. Then the area swells and heats up, a reaction called 'inflammation', which stimulates pain nerves. This takes about a day, so the pain comes late.
The second time hurts less
Repeat the same exercise and soreness drops
After going through an exercise once, the muscle is damaged less and is less sore the next time. That is because the muscle changes to protect itself against that exercise. This is called the 'repeated bout effect'.
A warning sign that is not soreness
Cola-colored urine is not normal soreness
Severe muscle pain and weakness after exercise together with urine that turns dark like cola is not normal soreness. It is a sign of 'rhabdomyolysis', where badly damaged muscle leaks its contents into the blood. These substances harm the kidneys, and the sooner it is treated, the lower the risk of permanent kidney damage.
Common misconception
Is soreness caused by lactic acid building up?
Lactic acid is made when muscles break down glucose for energy, and more of it is made during hard exercise. Many people blame next-day soreness on it, but lactic acid in the blood returns to normal levels within at most two hours after exercise. In fact, downhill running caused soreness even though lactic acid did not rise.
Summary
Soreness that comes late after unfamiliar exercise
DOMS is muscle aching and stiffness that comes about a day after unfamiliar exercise and fades within days. It is not caused by lactic acid left over after exercise; it is explained by tiny damage inside the muscle and the inflammation that follows. Related terms: eccentric contraction, lactic acid.