Gàidhealtachd
The Gàidhealtachd ([kɛːəl̪ˠt̪əxk] 
listen , English: Gaeldom), sometimes known as A' Ghàidhealtachd (English: The Gaeldom), usually refers to the Highlands and Islands of Scotland and especially the Scottish Gaelic-speaking culture of the area. The corresponding Irish word Gaeltacht refers strictly to Irish-speaking areas.
The term is also used to apply to the Scottish Gaelic-speaking Canadian areas of Nova Scotia and Glengarry County, Ontario.
"The Gàidhealtachd" is not truly interchangeable with "Scottish Highlands" as it refers to the language and not to the geography. Also, many parts of the highlands no longer have substantial Gaelic-speaking populations, and some parts of what is now thought of as the Highlands have long been Scots-speaking or English-speaking areas: Caithness, Cromarty, Grantown-on-Spey, Campbeltown, etc. Conversely, several Gaelic-speaking communities lie outwith the Highland, Argyll and Bute and Outer Hebrides council areas, for example Isle of Arran and parts of Perth and Kinross. Gàidhealtachd also increasingly refers to regions in Scotland where Scottish Gaelic is spoken as a first language by much of the population.
Galldachd (Gall-dom, Gall referring to a non-Gael) is often used for the Lowlands, although it is also notable that the Hebrides are known as Innse Gall due to the historical presence of Norsemen.
History
Until a few centuries ago, the Gàidhealtachd would have included much of modern-day Scotland north of the Firth of Forth and Galloway (up until the 18th century, and maybe later), excepting the Northern Isles, as evidenced by the prevalence of Gaelic-derived place names throughout most of Scotland and contemporary accounts. These include Dundee from the Gaelic Dùn Deagh, Inverness from Inbhir Nis, Argyll from Earra-Ghàidheal, Galloway from Gall-Ghaidhealaibh, and possibly Stirling from Sruighlea (though the etymology is uncertain; see article). Gaelic speakers from what would be considered traditionally English-speaking/non-Gaelic regions today included George Buchanan (from Stirlingshire), Robert the Bruce (from Galloway), and Margaret McMurray (from Ayrshire).
For historical reasons, including the influence of a Scots-speaking court in Edinburgh and the plantation of merchant burghs in much of the south and east, the Gàidhealtachd has been reduced massively to the present region of the Outer Hebrides, the Northwest Highlands, the Skye and Loch Alsh and Argyll and Bute, with small Gaelic populations existing in Glasgow and Edinburgh. The Highland Clearances of the 18th and 19th centuries also contributed to the decline of the language, as they reduced the population of the Scottish Highlands, which were predominantly Gaelic-speaking at the time.
Canadian Gàidhealtachd
Scottish Gaelic has survived among communities descended from immigrants in parts of Nova Scotia (especially Cape Breton Island), Glengarry County in present-day Ontario, Prince Edward Island, and Newfoundland and Labrador.