Airmail

Airmail (or air mail) is mail that is transported by aircraft. It typically arrives more quickly than surface mail, and usually costs more to send. Airmail may be the only option for sending mail to some destinations, such as overseas, if the mail cannot wait the time it would take to arrive by ship, sometimes weeks.
In June 2006 the United States Postal Service formally trademarked Air Mail (two words with capital first letters) along with Pony Express. On 14 May 2007, Air Mail was incorporated into the classification First Class Mail International.
A postal service may sometimes opt to transport some regular mail by air, perhaps because other transportation is unavailable. It is usually impossible to know this by examining an envelope, and such items are not considered "airmail." Generally, airmail would take a guaranteed and scheduled flight and arrive first, while air-speeded mail would wait for a non-guaranteed and merely available flight and would arrive later than normal airmail.
A letter sent via airmail may be called an aerogramme, aerogram, air letter or simply airmail letter. However, aerogramme and aerogram may also refer to a specific kind of airmail letter which is its own envelope; see aerogram.
The choice to send a letter by air is indicated either by a handwritten note on the envelope, by the use of special labels called airmail etiquettes, or by the use of specially-marked envelopes. Special airmail stamps may also be available, or required; the rules vary in different countries.
The study of airmail is known as aerophilately.
see also:
airmail etiquette, L-mail, mailplane, V-mail
bibliography:
Richard McP. Cabeen, Standard Handbook of Stamp Collecting (Collectors Club, 1979), pp. 207–221
online source:
British Postal Museum & Archive Information Sheet - Airmail