A thing that is used for holding a space for a replacement that may come in the future. The placeholder and the replacement don't have to be of the same type.
Examples of placeholders:
- You stand in the middle of a parking space during a busy day hour, waiting for an arriving friend to come and pack his car. You are the placeholder.
- In a software project, stubs are often used as placeholders for future code which is yet to be written.
- You solve a Rubik's Cube. In some solving techniques, parts of the cube are just placeholders for others parts that are moving toward their solved
- You write a document or a letter, but don't yet want to finish all the small details, so you put something in the place of a title / signature / header, etc.
- You step on a weight triggered land mine. Now, in order to save your life, the rescue team must put something on the mine which equals to your weight, so the mine's sensors would not detect a change and decide to detonate when you run for your life. Good luck.