Declarative programming is often defined as any style of programming that is not imperative. A number of other common definitions exist that attempt to give the term a definition other than simply contrasting it with imperative programming.
- A program that describes what computation should be performed and not how to compute it
- Any programming language that lacks side effects (or more specifically, is referentially transparent)
- A language with a clear correspondence to mathematical logic.
Declarative languages are intended to express what -- not how -- a program should accomplish.
Web based design (ie HTML/CSS) are both good examples. They describe instructions for the browser on how a webpage should be structured and represented visually but possess no ability of their own to produce the output. How browsers/clients consume their code can be wildly different as long as the output produces the intended result as per the W3C/WHATWG specify.
Serialization/deserialization and DSLs (Domain Specific Languages) like SQL are another good example. They are designed and intended to follow a strict standard of functionality regardless of what language/platform they are run on.
The key to a good declarative language is that it does a good job of describing a data/process in simple, finite terms with little room for subjective variation.
The key to a good implementation of a system that consumes/produces declarative code is consistency with any/all other systems that consume it.
Markdown is a good example of a useful declarative language that suffers from a poor initial specification/design. Markdown evolved organically in the wild because of its usefulness. Unfortunately, in the absence of a well defined specification, there are now a lot of different variations when it comes to implementation. Many notable developers have taken the standardization of Markdown as a personal mission but -- at best -- an official standard will only be accepted through the slow/painful process of survival of the fittest. Even then, variations will always exist to some extent.
Technically, there is a formal system for classifying languages called the Chomsky Hierarchy. It describes is mathematical terms the different types of programming grammars based on how they're parsed.
XML, JSON, and YAML generally fall under context-free or context-sensitive languages depending on how they're used.
Considering usage, I think Declarative is a better term to describe what you're looking for.