What term describes something that takes the place of another?

The term that describes something that takes the place of another is "substitute." A substitute serves as an equivalent or a proxy for the original item, fulfilling the same function or role.

In various contexts, such as cooking, education, or consumer goods, a substitute can provide a similar outcome or experience even though it may not be identical to the original. For example, if a certain ingredient is not available in a recipe, a cook might use a substitute ingredient that provides a comparable flavor or texture.

The other terms listed have different nuances. A placeholder typically indicates a temporary space or marker for something that will be provided later, while an alternative suggests an option or choice among various possibilities but does not specifically imply that it necessarily takes the place of something else. Replacement implies a direct exchange where one item is removed, and another takes its place, which may not always capture the essence of being interchangeable or functionally similar. Thus, "substitute" most accurately describes the idea of taking the place of something else.