The term is sometimes controversial. For starters, the Celts as an identifiable race are long gone, there are strong differences between traditional music in the different countries, and many of the similarities are due to more recent influences. There is also the notion that 'celtic' implies celtic mysticism and a particular influence in new-age music which has little to do with traditional music. In general, the strongest connections are between Irish and Scottish tradition and it is on these that Ceolas concentrates. Breton musicians frequently play in Irish or Scottish music and at least one modern Galician group (Milladoiro) sounds quite Irish. In Canada and the US, the traditions are much more mixed, and it is there that the term 'celtic' is most used, though it is also true that many groups from particular celtic regions play the music of another region too.
It is also worth remembering that even a term such as 'Irish traditional music' is a lumping together of many different styles, from the raw, Scottish-tinged music of Donegal to the lyrical, easy-going style of Clare and many other regional styles that are only partly compatible.
Thus, in the absence of a better term ('folk' or 'world' music are sometimes used but are much vaguer), and with the realisation of it's shortcomings, 'celtic' is what we use for Ceolas.