Definition of "air mail"

Universal Directory of Railway Officials and Railway Year Book, 1942-1943. The Directory Publishing Co. Ltd., [...] Westminster, S.W.1. [...] Price 20s. net.—[...] The compilers have used the air mails to a greater extent than ever before, and so have the railway administrations in transmitting their replies.

1942 November and December, “Railway Literature”, in Railway Magazine, page 383