Welsh/English Dictionary - By AccomoDATA

Common prefixes in Welsh place names. some prefixes, or stems appear time and again in Welsh place names.

Aber - This denotes the mouth of a river, a small stream, or a confluence of a river. It is usually followed by the name of the river. 

Examples are:
Abercraf            mouth of the river Craf (= garlic)
Abernant            mouth of a stream
Aberllynfi          mouth of the river Llynfi (= smooth)

Betws - a house of prayer. This is usually followed by the location of the prayer house or by its founder's name. 

Examples are:

Betws-y-Coed        prayer house in the wood
Betws Garmon        prayer house of Garmon
Betws Ifan          prayer house of Evan

Blaen - the source of a river or the head of a valley. (Blaenau is the plural form). 

Examples are:
Blaen-bedw          source of the river Bedw (= birch trees).
Blaen-cwm           head of the valley. This is a village situated at the
                    head of the Rhondda Valley.
Blaenau Ffestiniog  heads in the valleys in the land of Ffestin.
Bryn                a hill.
Brynarian           hill of silver.
Brynhyfryd          pleasant hill.
Bryncastell         castle hill
Bwlch               a pass or gap.
Bwlch-mawr          big pass.
Bwlch y Ddau Faen   pass of the two stones.
Bwlch y Mynydd      mountain pass

Capel               a chapel.
Capel Dewi          chapel of David, the patron Saint of Wales.
Capel Seion         chapel of Zion.
Capel Newydd        new chapel.
Castell             castle.
Castell Pictwn      picton's castle
Castell Coch        red castle
Castell Maen        stone castle

Coed                a wood
Coed-y-bryn         wood of the hill
Coed-yr-Ystrad      wood of the vale
Coed-y-Brenin       King's wood

Cwm                 a shallow valley.
Cwm-bach            a small valley
Cwm Hirnant         valley of the long stream.
Cwm Elai            valley of the elai river.

Dan                 below (also Tan)
Dan-y-coed          below the wood
Dan-y-graig         below the rock
Tanygrisiau         below the steps

Eglwys              a church
Eglwys-fach         little church
Eglwys y Drindod    Trinity church
Eglwys Wen          White church

Esgair              a ridge
Esgairdawe          a ridge of the river Tawe. (quiet)
Esgair Hirnant      ridge of the long stream.
Esgair Ddu          black ridge

Ffridd              a mountain pasture or sheep walk.
Ffridd Fawr         big mountain pasture.
Ffridd y Foel       mountain pasture of the bare hill.
Ffriddisaf          lower mountain pasture.
Ffynnon             a well or spring
Ffynnongroyw        clear spring or well
Ffynnon Taf         Taf's well.
Ffynnon-bedr        St Peter's well.

Geli                a grove.
Geli-aur            golden grove.
Gellilydan          wide grove.
Gellifelen          yellow grove.
Glan                a river or water bank.
Glan-llyn           the bank of a lake.
Glan-y-don          bank or shore of the wave.
Glanyrafon          bank of the river.
glyn                a glen or valley.
Glynteg             fair glen.
Glynebwy            glen of the river Ebwy.
Glyncorrwg          glen of the two Corrwg streams.
Gors                from Cors, meaning a bog, fen or marsh.
Gors Lydan          wide marsh.
Gorseinon           the fen of Einion.
Gors-goch           red marsh.

Hafod               a summer dwelling or summer pasture
Hafod yr Ynys       summer dwelling of the island.
Hafod-wen           fair summer dwelling or pasture.
Hafod Fraith        mottled summer pasture.

Llan - a church. This is usually followed by the name of the saint to whom the church is dedicated. 

Examples are:
Llanddewi           Church of St. David.
Llanbadrig          Church of St. Patrick.
Llanfair            Church of St. Mary.
                    (Lan- may also refer to a parish).
Llwyn               a bush or grove.
Llwyn-onn           ash grove.
Llwynypia           bush of the magpie.
Llwyndyrys          tangled grove.
Llyn                a lake or pool.
Llyn Helyg          lake of the willows.
Llyn Hywel          Hywel's lake.
Llyn Hir            long lake.

Maes                a field or plain.
Maes-glas           field of the bridge.
Maes-y-crugiau      field of the cairns.
Moel                a bare hill.
Moel Garegog        rocky, bare hill.
Moel Sych           dry, bare hill.
Moel y Llyn         bare hill of the lake.
Mynydd              a mountain.
Mynydd Coch         red mountain.
Mynydd Uchaf        highest mountain.
Mynydd Margam       Morgan's mountain.

Nant                a stream.
Nantglyn            a stream of the glen.
Nant-y-Pnndy        stream of the fulling mill.
Nantgarw            rough stream.

Pant                a hollow or valley.
Pantycelyn          hollow of the holly trees.
Pant yr Ynn         hollow of the ash trees.
Pantysgallog        hollow of thistles.
Pen                 top or end.
Pen-twyn            top of the hill.
Penrhos             end of the moorland.
Pentraeth           end of the beach.
Pentre              a village.
Pentre Isaf         lower village.
Pentre's Felin      village of the mill.
Pentre Bont         village of the bridge.
Pont                a bridge.
Pont Sian           Sian's bridge.
Pontaman            bridge over the river.
Pontgarreg          bridge of stone.
Pwll                a pool or pit.
Pwll-glas           blue pool.
Pwll-y-glaw         the rain pool.
Pwll defaid         sheeps pool or pit.

Rhiw                a slope or hillside.
Rhiwlas             green slope.
Rhiwderyn           birds hillside.
Rhiw Cilgwyn        hill of the white nook.
Rhos                moorland.
Rhos-yr-hafod       moorland of the summer dwelling.
Rhosbwlch           moorland of the pass.
Rhos-goch           red moorland.
Rhyd                a ford or stream.
Rhyd-y-foel         ford of the bare hill.
Rhydfach            little ford.
Rhyd-ddu            black ford.

Tal                 end or front.
Tal-y-bont          end of the bridge.
Talgarreg           end or front of the rock.
Talerddig           end of the little garden.
Tre                 a homestead or town (also Tref).
Treffynnon          homestead of the well.
Trefecca            homestead of Becca.
Trefdraeth          homestead or town of the beach.
Ty                  a house.
Tyddewi             house of St David.
Ty-hen              old house.
Ty-newydd           new house.
Tyn(from Tyddyn)    a small holding.
Tyn-y-pwll          a small holding of the pool.
Tyn-cwm             small holding in the valley.
Tyn-y-Pistyll       small holding near the well or spring.

Waun(fromGwaun)     moorland or meadow.
Waun Fawr           Large meadow.
Waun Lwyd           grey meadow or moorland.
Waunarlwydd         lord's meadow.

Ystrad              A wide bottomed valley.
Ystradmynach        monk's valley.
Ystradowen          Owen's valley.
Ystradyfodwg        valley of St Tyfodwg.