The best replacement for opacity is transparency.

The best alternatives to opacity are rated as following:
★★★★★  transparency
★★★★★  fill
★★★  flow
★★★  rgba
★★  infiltrate
★★  translucency
★  tint
★  transmittance
★  opaqueness
★  lucency
★  tappi opacity
★  visibility
★  fill photoshop
★  flow photoshop
★  optical density
  light transmission
  contrast ratio

Your alternative for opacity is not listed? Suggest!
Speaking about opacity, one probably shouldn't forget about hex colors.

Other search terms related to opacity and their ratings are:
★★★★★ hex colors
★★★★★ rgba
★★★★ background color
★★★★ rgb
★★ css background-color
★ png
background image
You have in mind a companion for opacity and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of opacity in all years. Click on graph for more.
opacity
transparency
fill