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Mesuriad byr o amser ydy eiliad. Mae'n uned sylfaenol y System Ryngwladol o Unedau (symbol: s). Fel arfer, caiff ei fesur gyda chloc neu oriawr ac ers diwedd yr 20ed ganrif gyda chlociau atomig.
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| Enghraifft o'r canlynol | unedau sylfaenol SI, uned amser, uned sylfaen UCUM, System Ryngwladol o Unedau gydag enw arbennig  |
|---|---|
| Math | cyfnod o amser |
| Rhan o | munud, system o unedau–centimetr–gram–eiliad, system o unedau MKS |
 Ffeiliau perthnasol ar Gomin Wicimedia | |
Mewn un eiliad o amser mae golau'n teithio(mewn gwactod) 299,792,458 metr, h.y. 300,000 km (~186,282 milltir).
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