Axiology, from the Greek axia (αξια, value, worth), is the study of value or quality. It is often thought to include ethics and aesthetics--philosophical fields that depend crucially on notions of value--and sometimes it is held to lay the groundwork for these fields, and thus to be similar to value theory and meta-ethics. The term was in the 19th and early 20th centuries, but in recent decades, value theory has tended to replace it in discussions of the nature of value or goodness in general.
One area in which research continues to be pursued is so-called formal axiology, or the attempt to lay out principles regarding value with mathematical rigor.
The term is also used sometimes in economics.
A popular work by Robert M. Pirsig, Zen and the Art of Motorcycle Maintenance, introduced the term "axiology" to a general audience, although not in any technical context.