I have heard Rambam's presentation of the mitzvos at the beginning of each set of halachos referred to as the כותרת (koteret).
What is the source for this usage?
Etymologically, the verb כתר (from whence we get the Hebrew noun, כתר, meaning "crown") means to surround, or encircle. In the Aramaic pael (as perhaps in Job 36:2) it can also have the added nuance of expecting something, or lying in wait. The derived noun, כֹתרת, appears in Tanakh in reference to the head of a pillar (so, for example, 1 Kings 7:16 and Jeremiah 52:22, etc) - perhaps because it concerned an ornamentation that "surrounded" the pillar, or "crowned" it in some fashion.
In the rabbinic literature, כותרת is used on several occasions to refer to the top-most part of a pillar, and you can see Jastrow for examples. From there, and based on the metaphorical relationship between works of literature and physical edifices, it also comes to denote the heading of a textual passage. This is a development that occurs post-Talmud, and is the reason why today, in Israeli Hebrew, a כותרת is a title.
I don't know what is the earliest instance in text of people referring to the material at the top of a section as its כותרת, but such passages are commonly described this way in speech - and not just in reference to the Rambam.