Seil (/ˈsiːl/; Scottish Gaelic: Saoil, Scottish Gaelic pronunciation: [ˈs̪ɯːl]) is one of the Slate Islands, located on the east side of the Firth of Lorn, 7 miles (11 kilometres) southwest of Oban, in Scotland. Seil has been linked to the mainland by bridge since the late 18th century.

The origins of the island's name are unclear and probably pre-Gaelic. It was a part of the kingdom of Dalriada in the 7th century but by the 16th century, Seil seemed to have been primarily agricultural in nature. It became part of the estates of the Breadalbane family and in the early 18th century they began to exploit the rich potential of the Neoproterozoic slate beds. The excavations from the island's quarries were exported all over the world during the course of the next two centuries. Today, the economy is largely dependent on agriculture and tourism.

The "dangerous seas" of the Firth of Lorn have claimed many lives and there are several shipwrecks in the vicinity of Seil. Kilbrandon Church has fine examples of stained glass windows and an association with St Brendan.

There is some linguistic continuity between the earliest and modern names for many of the larger islands surrounding Scotland. However, the derivations of many of these names are obscure "suggesting that they were coined very early on, some perhaps by the earliest settlers after the Ice Age." Even when names used both in the historic past and the present have some apparent meaning this may indicate a phonetic resemblance to an older name, but one that may be "so old and so linguistically and lexically opaque that we do not have any plausible referents for them."

The Ravenna Cosmography, which was compiled by an anonymous cleric in Ravenna around AD 700, mentions various Scottish island names. This document frequently used maps as a source of information and it has been possible to speculate about their modern equivalents based on assumptions about voyages made by early travellers 300–400 years prior to its creation. The island of Saponis mentioned in this list may refer to Seil.

Seil is probably a pre-Gaelic name, although a case has been made for a Norse derivation. It has also been argued that Seil could be the location of Hinba, an island associated with St Columba. Reasons include the island's association with St Brendan, its location on an inshore trade route from Antrim to the north and its suitability for a substantial settlement. The Muirbolcmar (great sea bag) referred to in texts about Hinba could refer to the Seil Sound and narrows at Clachan Bridge where the "bag" captures the rapidly flowing water that floods under the bridge. Rae, equating "Hinba" with the Gaelic Inbhir, notes that the adjacent mainland parish of Kilninver means "church of Inbhir" and suggests that the derivation of "Seil" may be of Scandinavian origin with similarities to the East Frisian place name Zijl or Syl meaning a "seep or passage of water". This, he proposes, could have been a Norse interpretation of Hinba/Inbhir. However, Mac an Tàilleir notes that Kilninver or Cill an Inbhir "appears to mean 'church by the river mouth', and an older form of Cill Fhionnbhair, 'Finbar's church' appears".

It has also been suggested that Seil may be the Innisibsolian referred to in the Chronicle of the Kings of Alba, which records a victory of the Scots over a Viking force during the time of Donald II in the 9th century. The name used in the 12th century Book of Leinster is Sóil.

The earliest comprehensive written list of Hebridean island names was undertaken by Donald Monro in his Description of the Western Isles of Scotland of 1549 in which Seil is listed. The modern spelling of "Seil" also appears in the 1654 Blaeu Atlas of Scotland.