Are there any authoritative definitions of "key stretching"?
In my practice (Smart Cards, often using DES and increasingly AES)


Key Expansion is often used to designate production of subkeys in a block cipher. This process is often a mere bit extraction, as part of the algorithm's Key Schedule.
Key Diversification is, almost exclusively, the process of producing a device key from its serial number (or other identifier) and a master key. This transformation must be such that compromise of the key of some devices does not compromise the master key, or the key of other devices.
Key Derivation is a general term for producing a key from another key. It is often used for Key Diversification, or the process of expanding one key to several keys of different function (e.g. a confidentiality key for CBC encryption, and an integrity key for CBC-MAC), or the process of obtaining a session key from a long-term key. The term Key Derivation tends to imply that the derived key does not leak from the original key (as illustrated in Key Diversification).


Further, I would understand Key Strengthening or Key Hardening (exclusively), or Key Stretching (less specifically) as a purposely compute-intensive transformation of a key, effectively increasing its resistance to brute force search by a parameterizable factor, e.g. in PBKDF2 or Scrypt. Key Stretching is attested in this meaning, as stated in the question. But I have also seen Key Stretching used for the process of obtaining several keys from a single one, without intend to slowdown a brute force search.

And I would understand Key Salting as the ages-old process of adding (public or mildly secret) salt to a key (of low-entropy such as a password), so that at least identical keys do not degenerate into identical cryptograms, without indication of the deliberate introduction of a compute-intensive transformation as in Key Strengthening.

I am not an authority, English is my second language, and I have somewhat changed my mind on the usual meaning of Key Stretching.