By Colette Hume BBC Wales Education Correspondent

What's got more than seven million words, 3000 pages and has taken 81 years to complete - it's quite a question and the answer is the definitive Welsh dictionary.

The last of four volumes has finally been completed by staff at the University of Wales dictionary unit and is being published on Thursday.

Controversially, the letter 'Z' which officially is not part of the Welsh alphabet, is being included and there are three words under the heading including Zwingliaidd, which according to the editors, describes the doctrine of the Swiss theologian Ulrich Zwingli.

Co-editor Patrick Donovan: 'Quick turnaround'

The task began back in 1921 with the first volume of the dictionary was published in 1950. So why did it take so long?

Co-editor Patrick Donovan says eighty one years is a quick turnaround: "Our dictionary was modelled on the large Oxford English dictionary.

"They took in calendar years fifty years time but, of course, they had a number of different teams working on the dictionary.

"Two at the beginning from 1888 onwards and it the 1920's they had three teams working on the dictionary.

"So, in fact, our dictionary has not taken any longer than any other comparable dictionary."

The earliest word 'Ynys�, meaning island, was first recorded in 601, but there are also a huge collection of newer words to describe the worldwide web.

Prestige value

For rugby fans there's the word 'trosgais' which means a converted try.

Reflecting the international situation the word 'taflegryn' describes missile.

There are words ranging from just one letter in length to the 21 letter long 'cyfrwngddarostyngedigaeth' - a 15th-century word meaning 'intercession'.

Language expert David Crystal said: "What a dictionary does is it gives a kind of prestige or symbolic value to what people say.

"You can look things up in a dictionary, it's there, you can pull it down off the shelf and look at it and check your feelings about how the language is developing.

The dictionary was launched at the Welsh Assembly

"In that sense the dictionary not only gives a permanent presence to a language, it marks an important stage in that language's development.

The final volume was launched by First Minister Rhodri Morgan at the Welsh Assembly in Cardiff on Thursday.

The dictionary has a personal connection for Mr Morgan.

His late father Professor T J Morgan, who was at one time registrar of the University of Wales, served for many years on the dictionary's editorial board while Professor of Welsh at Swansea University.