Is an adapter from a plug (say a NEMA 14-50) to a different socket (say a NEMA 6-20 or L14-20, or even a NEMA 6-50 for that matter) considered an extension cord by the 2014 NEC? If it isn't, then what type of device would it be, and would it matter if it was a listed assembly vs. a field-fabricated assembly?
As to the use of "extension cord" in the NEC, see 240.5(B)(3)/(4) for an example:
(3) Extension Cord Sets. Flexible cord used in listed extension cord sets shall be considered to be protected when applied within the extension cord listing requirements.
(4) Field Assembled Extension Cord Sets. Flexible cord used in extension cords made with separately listed and installed components shall be permitted to be supplied by a branch circuit in accordance with the following:
20-ampere circuits - 16 AWG and larger