Indelible means permanent, as in indelible ink. If it gets on anything, you can’t wash it off. It’s stuck there permanently. A perfect example of indelible ink is India ink, which I discussed in my first A to Z Challenge back in April 2012. (Be sure to get ready for this year’s Challenge!)
When I was leading training classes, we had two different kinds of markers: erasable for the white board, permanent for the flipchart. Many’s the time an instructor managed to use the marker for the flipchart on the white board. We discovered that the secret to cleaning it off the board was to scribble over it with an erasable marker, then erase the scribbling. Presto! The permanent mark was gone!
Back in the paleolithic area, before we had PowerPoint, we used transparencies on an overhead projector and projected them on a screen. One day one of my colleagues was so involved with talking about something that she started writing on the screen in indelible marker. Fortunately, she caught herself before she wrote too much, and they were able to get all but a red line off the screen, which we decided we’d leave there to remind all the instructors not to write on the screens. It didn’t happen again.