One of two sub-branches of the Celtic languages, also known as "Brythonic", which now consists of Welsh and Breton. Cornish, which is now extinct, was also a Brythonic language. It is commonly believed that the languages of ancient Gaul and Celtic Britain were also Brythonic, and most likely somewhat similar to modern Welsh.
The designation of "P-Celtic" arose because one original Indo-European sound manifested itself as a "p" in the Brythonic languages. An example is "four" in Welsh, "pedwar", as opposed to "ceathair" in Irish.
See also Q-Celtic.
Many thanks to Gritchka for the "four" example.