The online definition of the word "ephemera" is:
Quote:
paper items (as posters, broadsides, and tickets) that were originally meant to be discarded after use but have since become collectibles.
So in its relation to "postal ephemera" it is typically defining things like labels, forms, etc., that have legitimate use as part of the mailing system, but are not usually meant to be retained by collectors as having any lasting value.
Since philatelic collectibles such as covers, first day ceremony programs, etc., are meant to be collected, they are (in my mind) not postal ephemera. Rather, the definition would more appropriately be placed on things like USPS labels (i.e. priority mail, etc.), airmail etiquettes, advertising pieces, posters or even post office forms used for any number of situations. Such items, although a necessary part of postal operations in general, were never produced with the intent of ever becoming a collectible, yet since they relate to the post office in one form or another, those who choose to collect them would typically refer to the items as "postal ephemera".