It contains all 15 mountains in Wales over 3000 feet high, including the country's highest, Snowdon (Yr Wyddfa), which is 1,085 metres (3,560 ft) tall.

The national park has an area of 823 square miles (2,130 km2) (the fourth-largest in the UK), and covers most of central and southern Gwynedd and the western part of Conwy County Borough.

[5][6][7][8] Before the boundaries of the national park were designated, "Snowdonia" was generally used to refer to a smaller upland area of northern Gwynedd centred on the Snowdon massif.

[9] The Mountains of Snowdonia by H. Carr & G. Lister (1925) defines "Eryri" as "composed of the two cantrefs of Arfon and Arllechwedd, and the two commotes of Nant Conwy and Eifionydd",[10] which corresponds to Caernarfonshire with the exception of southwest Llŷn and the Creuddyn Peninsula.

Following a two-year transition period, the authority announced in November 2024, the change would be kept due to its success in gaining support and adoption by many businesses and media.

The last ice age ended only just over 11,500 years ago, leaving features attractive to visitors, which have also played a part in the development of geological science and continue to provide a focus for educational visits.

There are smaller areas of Silurian age sedimentary rocks in the south and northeast and of Cenozoic era strata on the Cardigan Bay coast, though the latter are concealed by more recent deposits.

Low grade metamorphism of Cambrian and Ordovician mudstones has resulted in the slates, the extraction of which once formed the mainstay of the area's economy.

Part of the east of the national park is within the upper Dee (Dyfrydwy) catchment and includes Bala Lake, the largest natural water body in Wales.

Marchlyn Mawr reservoir and Ffestiniog Power Station's Llyn Stwlan are two cases where natural tarns have been dammed as part of pumped storage hydro-electric schemes.

[25] The Mawddach and Dwyryd estuaries form the largest indentations, and have large expanses of intertidal sands and coastal marsh which are especially important for wildlife: see #Natural history.

The northern tip of the national park extends to the north coast of Wales at Penmaen-bach Point, west of Conwy, where precipitous cliffs have led to the road and railway tunnels.

[32] Similarly the local economies of the towns of Conwy, Bethesda, and Llanberis in the north are inseparably linked to the national park as they provide multiple visitor services.

Other sizeable villages are Llanuwchllyn at the southwest end of Bala Lake (Llyn Tegid), Dyffryn Ardudwy, Corris, Trawsfynydd, Llanbedr, Trefriw and Dolwyddelan.

Six primary routes serve Snowdonia, the busiest of which is the A55, a dual carriageway which runs along the north coast and provides strategic road access to the northern part of the national park.

The A487 loops around the northwest of the park from Bangor via Caernarfon to Porthmadog before turning in land to meet the A470 east of Maentwrog.

The A5 was built as a mail coach road by Thomas Telford between London and Holyhead; it enters the park near Pentrefoelas and leaves it near Bethesda.

The network was relaunched in July 2022 with this new brand to reflect the National Park's promotion of Welsh place names.

Together with the TrawsCymru network of buses, this provides a car-free option for tourists and locals wishing to travel across the National Park.

Roads are known to have connected with Segontium (Caernarfon) and Deva Victrix (Chester) and include the northern reaches of Sarn Helen.

[40] The Snowdonia Society is a registered charity formed in 1967; it is a voluntary group of people with an interest in the area and its protection.

Amory Lovins led the successful 1970s opposition to stop Rio Tinto digging up the area for a massive mine.

[41] The park's entire coastline is a Special Area of Conservation, which runs from the Llŷn Peninsula down the mid-Wales coast, the latter containing valuable sand dune systems.

It can form massive towering growths and has a companion fungus that grows on its roots producing toxins that are poisonous to any local flora and fauna for a seven-year period after the Rhododendron infestations have been eradicated.

[46] There are numerous SSSIs within the park, the most extensive of which are Snowdonia, Migneint-Arenig-Dduallt, Morfa Harlech, Rhinog, Berwyn, Cadair Idris, Llyn Tegid, Aber Mawddach / Mawddach Estuary, Dyfi, Morfa Dyffryn, Moel Hebog, Coedydd Dyffryn Ffestiniog and Coedydd Nanmor.

The park boundaries are drawn such that much of the landscape affected by slate quarrying and mining lies immediately outside of the designated area.

Among hikers' favourites are Y Garn (east of Llanberis) along the ridge to Elidir Fawr; Mynydd Tal-y-Mignedd (west of Snowdon) along the Nantlle Ridge to Mynydd Drws-y-Coed; Moelwyn Mawr (west of Blaenau Ffestiniog); and Pen Llithrig y Wrach north of Capel Curig.

The Cambrian Way is a long-distance trail between Cardiff and Conwy that stays almost entirely within the national park[55] from Mallwyd northwards.

[58] In 2020, an e-petition calling for the removal of the English name was put forward to the Senedd, but rejected as responsibility lies with the Snowdonia National Park Authority.

[60][61] The authority was already considering its language policy, but stated that the petition encouraged it to take "decisive action", and it resolved to use Welsh names as far as legally possible in November 2022;[62] it must continue to use "Snowdonia" in statutory documents.