The Enchantix is a form for fully-fledged fairies on the animated TV series, Winx Club.
All Complete Enchantix Forms (note the chokers[or chain] around each girl's neck)
In order for a fairy to earn her Enchantix, she has to save someone from her home realm and show great sacrifice in doing so. When in her Enchantix stage, the fairy's attacks are much stronger, she can decrease in size and break dark spells with her fairy dust which is in a vial around her neck. Fairies get their fairy dust in Enchantix form and also a set of strong, stylish wings which help them fly faster.
Bloom and Tecna are the only known exceptions to this rule. By believing in her power to defeat Evil (after meditating with the sorceress Maiya on Pyros) Bloom was able to will herself into Enchantix. Tecna did not save anyone from her own realm, she saved the realm of Tides by forcing her way through the portal to the Omega Dimension and thus shutting the portal. It is said that witches that transfer from darkness to light can earn enchantix, eg. Mirta seen in Winx form. Once a fairy earns her Enchantix, she cannot transform into her Winx form again.
Passion for another's life could be a key factor in triggering a fairy's Enchantix status. i.e. When Galatea volunteered to perish in the Alfea library fire to give Musa a chance to escape, Musa refused to leave her friend. Her love for her friend was so strong that it was enough to earn her her Enchantix. Also, even though her Enchantix powers were incomplete when she earned them, in the RAI version, Bloom's passion to defeat Evil, and her undying hopes and dreams to find her birth parents may have been what helped her earn her own Enchantix powers.
There are similar but different battle cries between the original Italian and the 4Kids Versions. In the 4Kids version, after they've earned their Enchantix powers, the Winx girls shout, "Let's go, Enchantix!" or, on odd occasions, just, "Go Enchantix!" In the original Italian version, the battle cry is "Winx Enchantix!"