What is the definition or description of: metaplasia?

Change...: Metaplasia is thought to be an adaptation of the body that replaces one type of cell with another, usually caused by some sort of stress. Metaplasia is not cancerous, per se, but depending on the circumstances, may be a very early step in the process of a normal cell becoming cancerous.

Changed tissue: "metaplasia" refers to one tissue type undergoing a change to another, such as bronchial lining (which is usually a single layer of cells, "respiratory epithelium" becoming multi-layered and heavily keratinized, so-called squamous metaplasia. This is common in smokers. Other such changes can produce tissue such as bone in abnormal sites.