The Thames, from Middle English Temese, is derived from the Brittonic Celtic name for the river, Tamesas (from *tamēssa), recorded in Latin as Tamesis and yielding modern Welsh Tafwys "Thames". The name may have meant "dark" and can be compared to other cognates such as Russian темно (Proto-Slavic *tĭmĭnŭ), Lithuanian tamsi "dark", Latvian tumsa "darkness", Sanskrit tamas and Welsh tywyll "darkness" (Proto-Celtic *temeslos) and Middle Irish teimen "dark grey". The same origin is shared by countless other river names, spread across Britain, such as the River Tamar at the border of Devon and Cornwall, several rivers named Tame in the Midlands and North Yorkshire, the Tavy on Dartmoor, the Team of the North East, the Teifi and Teme of Wales, the Teviot in the Scottish Borders, as well as one of the Thames' tributaries called the Thame.
Kenneth H. Jackson has proposed that the name of the Thames is not Indo-European (and of unknown meaning), while Peter Kitson suggested that it is Indo-European but originated before the Celts and has a name indicating "muddiness" from a root *tā-, 'melt'.
Indirect evidence for the antiquity of the name 'Thames' is provided by a Roman potsherd found at Oxford, bearing the inscription Tamesubugus fecit (Tamesubugus made [this]). It is believed that Tamesubugus' name was derived from that of the river. Tamese was referred to as a place, not a river in the Ravenna Cosmography (c. AD 700).
The river's name has always been pronounced with a simple t /t/; the Middle English spelling was typically Temese and the Brittonic form Tamesis. A similar spelling from 1210, "Tamisiam", is found in the Magna Carta.
The Thames through Oxford is sometimes called the Isis. Historically, and especially in Victorian times, gazetteers and cartographers insisted that the entire river was correctly named the Isis from its source down to Dorchester on Thames and that only from this point, where the river meets the Thame and becomes the "Thame-isis" (supposedly subsequently abbreviated to Thames) should it be so called. Ordnance Survey maps still label the Thames as "River Thames or Isis" down to Dorchester. However, since the early 20th century this distinction has been lost in common usage outside of Oxford, and some historians suggest the name Isis is nothing more than a truncation of Tamesis, the Latin name for the Thames. Sculptures titled Tamesis and Isis by Anne Seymour Damer can be found on the bridge at Henley-on-Thames, Oxfordshire (the original terracotta and plaster models were exhibited at the Royal Academy, London, in 1785. They are now on show at the River and Rowing Museum in Henley).
Richard Coates suggests that while the river was as a whole called the Thames, part of it, where it was too wide to ford, was called *(p)lowonida. This gave the name to a settlement on its banks, which became known as Londinium, from the Indo-European roots *pleu- "flow" and *-nedi "river" meaning something like the flowing river or the wide flowing unfordable river.
For merchant seamen, the Thames has long been just the "London River". Londoners often refer to it simply as "the river" in expressions such as "south of the river".
The river gives its name to three informal areas: the Thames Valley, a region of England around the river between Oxford and West London; the Thames Gateway; and the greatly overlapping Thames Estuary around the tidal Thames to the east of London and including the waterway itself. Thames Valley Police is a formal body that takes its name from the river, covering three counties. In non-administrative use, the river's name is used in those of Thames Valley University, Thames Water, Thames Television, publishing company Thames & Hudson, Thameslink (north-south railways passing through central London) and South Thames College. An example of its use in the names of historic entities is the Thames Ironworks and Shipbuilding Company.