replaceˈment noun

1. The act of replacing
2. A person or thing that takes the place of another
3. The occurrence of a face or faces in the position where the principal figure would have a corner or edge (crystallography)
4. The process by which one mineral gradually forms from another in crystalline form by solution and redeposition (geology)

replacˈer noun

A substitute

replaceable hydrogen noun

Hydrogen atoms that can be replaced in an acid by metals to form salts