(adj.) Characteristic of a particular piece of data if its value or format is not normally visible to functions that use it. For example, the input_token parameter to gss_init_sec_context() is opaque to the application, but significant to the GSS-API. Similarly, the input_message parameter to gss_wrap() is opaque to the GSS-API but important to the application that does the wrapping.
adj. Refers to a data type which is intended to be abstract: code using the type is not supposed to know how the type is implemented (whether it is a simple type or a structure, and if a structure, what fields it contains). See question 2.4.