Just to throw in useless information actinic light used to mean light that would expose photographic (B&W) film - ie the green or blue parts of the spectrum. A non-actinic light was a safe light - ie a red light in a darkroom.
But now as tkeracer619 says, it customarily refers to light that peaks at 420 nm - violet. Blue is defined as peaking at 470 nm.
The ATI Blue+ is advertised as having a combination of 460 and 420. The ATI True Actinic is spec'd as peaking at 421 nm.