The Thames, from Middle English Temese, is derived from the Celtic name for the river, Tamesas (from *tamessa), recorded in Latin as Tamesis and underlying modern Welsh Tafwys "Thames". The name probably meant "dark" and can be compared to other cognates such as Irish teimheal and Welsh tywyll "darkness" (PC *temeslos) and Middle Irish teimen "dark gray", though Richard Coates mentions other theories: Kenneth Jackson's that it is non Indo-European (and of unknown meaning), and Peter Kitson's that it is IE but pre-Celtic, and has a name indicating muddiness from a root *tã-, 'melt'.
The river's name has always been pronounced with a simple t; the Middle English spelling was typically Temese and Celtic Tamesis. The th lends an air of Greek to the name and was added during the Renaissance, possibly to reflect or support a belief that the name was derived from River Thyamis in the Epirus region of Greece, whence early Celtic tribes were erroneously thought to have migrated.
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's name was derived from that of the river.
The part of the Thames running through Oxford is often given the name the River Isis, although historically, and especially in Victorian times, gazetteers and cartographers insisted that the entire river was correctly named the River Isis from its source until Dorchester-on-Thames. Only at this point, where the river meets the River Thame and becomes the "Thame-isis" (subsequently abbreviated to Thames) should it be so-called; current Ordnance Survey maps still label the Thames as "River Thames or Isis" until Dorchester. However since the early 20th century, this distinction has been lost in common usage outside Oxford, and some historians suggest the name Isis—although possibly named after the Egyptian goddess of that name—is nothing more than a contraction of Tamesis, the Latin (or pre-Roman Celtic) name for the Thames.
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'.
One of the major resources provided by the Thames is drinking water provided by Thames Water whose area of responsibility covers the length of the River Thames. The Thames Water Ring Main is the main distribution mechanism for water in London with one major loop linking the Hampton, Walton, Ashford and Kempton Water Treatment Works to central London.
In the past, commercial activities on the Thames included fishing (particularly eel trapping), coppicing willows which provided wood for many purposes including osiers, and running watermills for flour and paper production and metal beating. These activities have disappeared, although there was a proposal to build a hydro plant at Romney Lock to power Windsor Castle. As of January 2008, this scheme appears to have been abandoned.
The Thames is popular for riverside housing whether in high rise flats in central London or chalets on the banks and islands up stream. The river has its own residents dwelling on houseboats, typically around Brentford and Tagg's Island. One of the major industries on the river now is tourism. In London there are many sightseeing tours in tourist boats, past the more famous riverside attractions such as the Houses of Parliament and the Tower of London as well as regular riverboat services provided by London River Services. Along the course of the river a number of small private companies offer river trips at tourist sites such as Hampton Court, Windsor and Oxford. Many companies also provide boat hire and accommodation on the river.