When one or component is painted fully or partially on top of another we say it
obscures the previous object. Some people use the
astronomical term occults. Cover is another favourite. The Xlib people make a subtle distinction
between obscure and occlude.
Occlude refers to the stacking order, whether or not the windows are actually
displayed, where obscures implies actual intersection on the screen. However, the
Xlib definition contradicts itself, so it may be the other way around. I personally
don’t like the term occlusion because it has the
connotation of congealed hair stopping up a drain, or ill fitting dentures rather
than simply eclipsing.