A round tail (sometimes called a round pin in its narrower variant) ends with a continuous arcing curve — no corners, no notches, no flat edges. The whole back of the board comes to a single rounded point (round pin) or a wider rounded crescent (round tail proper).
What it does on a surfboard
The round shape holds a line predictably through fast sections and turns with a smooth, drawn-out arc rather than a snappy pivot. It doesn't release as quickly as a square tail or as freely as a swallow, but it gives the most consistent grip across a wide range of speeds and angles.
It's a standard tail on mid-lengths, longboards, step-ups, and many modern HPSBs that prioritize smooth flow over radical pivot. Round-pin variants (narrower) trade a bit of rail surface for more drawn-out turning radius — useful in bigger, fast surf where you want the board to track instead of skitter.
What to look for
On a model page, "round tail" or "round-pin tail" indicates this curve family. Lundquist offers round and round-pin as default or alt tail variants on most shortboards (excepting models specifically tuned around squash or swallow geometry).