I've noticed that certain security professionals use the term "ciphertext" while others use "cryptogram". Is there any meaningful difference between those terms? Or can they be used interchangeably?

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    Have never seen cryptogram in use. – schroeder Dec 29 '17 at 23:53

They're grammatically different for sure, because cryptogram is exclusively a count noun, while ciphertext can be either a count or a mass noun.

Also, I can document that the term cryptogram is sometimes used in senses where it's definitely not synonymous with ciphertext. For example, in the v2.2 specification of the Keyak cipher (a candidate in the ongoing CAESAR competition), p. 3:

The mode Motorist supports the authenticated encryption of sequences of messages in sessions. During a session, it processes messages and cryptograms. A message consists of a plaintext and possible associated data (called metadata in the remainder of this document). For each message, it wraps it by enciphering the plaintext into a ciphertext and computing a tag over the full sequence of messages. A cryptogram consists of a ciphertext, possible metadata and a tag. For each cryptogram, it unwraps it by deciphering the ciphertext into a plaintext, verifying the tag, and returning the plaintext if the tag is valid. A message can also consist of metadata alone and the corresponding cryptogram does not have any ciphertext.

Clearly the authors here use ciphertext and cryptogram in different senses. The last sentence tells us that in their usage, there is such a thing as a cryptogram without any ciphertext.

I certainly don't mean to imply that this is by any means an universally accepted usage, however. In many fields, different authors often use slightly different definitions of the same terms. The best advice is to always try to discern the authors' meaning in the context they use the terms, and not to try to impose some external definition they might not be heeding.

So perhaps counterintuitively, the answer is "yes" to both of your questions:

Is there any meaningful difference between those terms? Or can they be used interchangeably?

There are many contexts, perhaps even most, where the choice between those two words doesn't matter. And a few contexts where it does, like the Keyak specification.

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