The overround is what a market's implied probabilities add up to. On a standard two-way market that total is around 104.76%, not 100%.
The word is used two ways, which is the main source of confusion. Strictly, the overround is the total (104.76%); the margin is the amount that total exceeds 100% by (4.76%). Plenty of people use "overround" for the excess itself, and in practice you have to read which one is meant from context. The safest habit is to state the number you mean: "a booksum of 104.76%" or "a 4.76% margin" leaves nothing to interpret.
A third number gets tangled in with these two and genuinely differs. Hold is the margin as a share of the overround — 4.76 ÷ 104.76 = 4.55% — and it is the theoretical proportion of staked money retained. On a tight market margin and hold round to the same figure, which is why they get swapped. On a wide one they separate: a 20% margin is a 16.7% hold. Quoting one as the other always overstates.
Overround is not constant. It varies with how confident a market is and how much money is on it, so a heavily-traded match-winner market runs a low single-digit total while a thin, exotic market can run several times that. That variation is exactly why two markets cannot be compared until the margin has been removed from each — see why odds add up to more than 100% for the full treatment, and de-vigging for the removal step.
An overround below 100% is possible in principle and unusual in practice: it means the prices, taken together, imply less than one certain outcome. Our tools report that case rather than pretending it away, because a market with no margin has nothing to remove.