The opacity property

Objects also have a property named opacity.  (Opacity is the reverse of transparency.)

Can have any value ranging from 0 to 1.

If the value is 0, the object is completely transparent (invisible).

If the value is 1, the object is 100% opaque (non transparent) making it completely visible. 

Values between 0 and 1 cause it to have some degree of opacity or transparency.