Definition of "firth"

firth¹

/fɜːθ/

noun
plural firths
1. An arm or inlet of the sea; a river estuary.
  Quotations
  The descent continues, still more steeply to Dundee (Tay Bridge), and approaching from the bridge itself this sharp descent gives the curious appearance that the station is below the level of the firth.
  1947 January and February, O. S. Nock, “"The Aberdonian" in Wartime”, in Railway Magazine, page 7
  They drove on, every rise in the road lifting their sightline clear of the drystone dykes along the roadside, gifting glimpses of the firth and the islands, the blue peaks of Arran.
  2022, Liam McIlvanney, The Heretic, page 440

firth²

noun
plural firths
1. (chiefly Northern England, Scotland) Alternative form of frith (“a forest used for hunting; a (small) wood; wooded country; land covered mainly by brushwood”)