Active Camouflage is a special ability appearing ever since the original Halo game. At first it was merely a power-up but has evolved since then. I miss the days of Halo 2 where I'd use it constantly because it would be easy to find and gave near-perfect invisibility under a short time frame.
It's biggest change happened during Halo: Reach where you could activate it anytime you wanted if you chose it as your armor ability. Except it's very imperfect as the radar gives a big hint, and moving or attacking will make you more visible. You almost have to wait than find enemies.
I liked it a little better in Halo 4. It wasn't the greatest ability for offense, but if you managed to run away it wouldn't be hard to use it to hide from a brutish opponent.