CA (Conformity Assessment) is any activity which results in determining whether a product or other object corresponds to the requirements contained in a specification.
A specification, typically but not exclusively a standard, is a technical description of the characteristics which are required to be fulfilled by some object. These objects may be products (which in this context include services), processes, bodies, people or systems (management systems, for example).
Officially, CA is the “demonstration that specified requirements relating to a product, process, system, person or body are fulfilled”. Note that the phrase “conformity assessment” does not limit or classify the activity in any way – a consumer who buys an appliance and at home checks that it conforms to a technical description may be said to be carrying out “conformity assessment”.