Whorf (1956: 88ff) apud Halliday & Matthiessen (1999: 27):
A covert category may also be termed a cryptotype, a name which calls attention to the rather hidden, cryptic nature of such word-groups, especially when they are not strongly contrasted in idea, nor marked by frequently occurring reactances such as pronouns. They easily escape notice and may be hard to define, and yet may have profound influence on linguistic behaviour.