The word 'Winx' has two completely different concepts and meanings in the series, depending on the dub.
Winx is also the unofficial name for the Season 1 transformations.
The word Winx is original the word 'wings' or 'wing' with 'gs/g' replaced with an 'X'. It is one of the trademarks of this series.
Previous Transformation: None
Next Transformation: Charmix
In the RAI dub, it is simply the name of the Winx Club created by Bloom. By the end of season one however Faragonda adds the word to the vocabulary of the Fairies and of the Magic Dimension to make it become the definition of a Modern Fairy.
The word Winx has a much more significant meaning and role in the 4kids dub. Winx is a form of magical energy and magical concept for fairies. Basically, it is a fairy's magical power. A fairy draws their Winx from their magical source of power, e.g. Flora draws her Winx from flowers and plants, and Musa draws her Winx from music (as shown in "Magic In My Heart", when she draws a large amount of power from everyone singing).
In the Nickelodeon dub, the transformation is renamed "Magic Winx Charmix" and the transformation sequences for each fairy are reanimated. However, when Nick aired season 3 of Winx Club, the transformation returned to Rai's Magic Winx.