Transparency is useful in plots when you need an unobstructed view of multiple components of one plot, or simply want to lighten a single plot component against a white background. The Wolfram Language uses the graphics directive Opacity to apply transparency to graphics objects. Opacity can be used with most visualization functions.

Here is a 2D plot:

Use the PlotStyle option with Opacity to make the plot 70% transparent:

Here is a 3D plot:

Make the plot 60% transparent:

Add different transparency settings for plots of multiple functions:

Here is a 2D region determined by two inequalities:

Make the plot 50% transparent:

Use Opacity to see the inner parts of a plot:

Use the ContourStyle option of ContourPlot3D with Opacity:

You can also add transparency to graphics.

Make a 75% transparent disk:

Opacity can have values between 0 and 1, with 0 corresponding to perfect transparency:

Use Opacity with a color:

Opacity interacts well with other Wolfram Language functions: