Aiden (with an "e") is the most popular U.S. spelling variation of Aidan (with an “a”). Aidan is the traditional Irish spelling and an anglicized form of the ancient Gaelic name "Áedán" or "Aodhán." These names are diminutives of Áed from "Aodh" (fire), and therefore mean "little fire." In Irish mythology, Áed was the god of fire and god of the underworld. Aodhán is a name that was also borne by several ancient High Kings of Ireland as well as important early Irish saints. The older Gaelic versions of the name were relatively common during Antiquity and the early medieval period. Probably the most famous Irishman named Aidan was the 7th century Saint Aidan of Iona (Scotland) who also established a monastery in Northumbria (England) which brought Christianity to the pagan Saxons. The spelling of Aidan is most popular in Ireland and Northern Ireland, but the Northern Irish still use the more ancient variation of Aodhán with some frequency. Aiden with an "e" is an Americanized spelling.
The name Aiden did not show up on the U.S. popularity charts until very recently (1995), and even then it limped into the 934th position - so we'd barely call it familiar 20 years ago. However, very rapidly, its climb to the top of the charts in recent years was enough to make you dizzy. In less than 10 years, Aiden achieved Top 100 status, and then quickly thereafter (2008), it hit the Top 20. In 2010 it became a Top 10 favorite nationwide. Additionally, this is a name that Americans have been very creative with in terms of spelling. Other variations on the Top 1000 list of American baby boy names are: Aidan, Aidyn, Ayden, Aydan, Aydin, Aaden, and Aden (it's gotten a little out of hand). The preferred spelling in Ireland is actually Aidan, but in the U.S. it’s Aiden with an “e”. The name's popularity in America is attributed to an on-going interest in Irish names as well as the popularity of "a-den" sounding names (think: Kaden, Hayden, Jayden, Brayden, etc).