Yes, it is a trademark, and was a patent licensed to Nike - US
number 4219945 - the patent appears to have expired during the year
1997, allowing other companies to develop their own "Air"
mechanisms. The mechanism/technology of the Nike Air capsule was
invented by Marion F Rudy, and licensed to Nike. If you are oblivious
to what a Nike Air bubble is: it's described within it's patent
as a polyurethane plastic capsule, filled with inert gas, which
is placed within the midsole or outsole of footwear.
Marion F Rudy is also responsible for another US patent - number
4340626 - which is essential to the functionality of the Nike AIR
capsule. This patent covers the technique in which the gas is trapped
within the capsule.