Whorf (1956: 88ff) apud Halliday & Matthiessen (1999: 26-7):
An overt category is a category having a formal mark which is present (with only infrequent exceptions) in every sentence containing a member [instance] of the category. The mark need not be part of the same word to which the category may be said to be attached in a paradigmatic sense; i.e. it need not be a suffix, prefix, vowel change, or other ‘inflection’, but may be a detached word or a certain patterning of the whole sentence …