Need help in finding a component thing

i was fooling around, trying some things last night and hit something that gave me a list of controls. at the bottom of the list of whatever i hit there was an "A' followed by a slider. i presume this was an alpha channel thing where you could control the transparency of an image. but i cant find it today. it sort of looked like:

A: _________________________

and you could move the slider along that path.

i was just testing a lot of things and came across whatever that is and thought i'd come back to it later, but now i cant find the thing.

anyone know what that is? and how do i bring it back up? i've looked at every single component in the list and cant find anything that is the same. i've looked on the user side at everything i can think of, including the HDRI lighting, but still cant find it.

craig

It's the Alpha Channel