What is the official, generally accepted definition of "lightweight crypto"? I frequently stumble upon the term &quot;lightweight crypto&quot;. It tends to be used is a for special algorithms that use less resources, for constrained systems. Yet, it's somewhat unclear to me where to draw the line between &quot;regular algorithms&quot; and &quot;special algorithms&quot;.
Since even some well-known cryptographers use the term &quot;lightweight cryptography&quot;, I assume there must be an official, generally accepted definition for it. Yet, I fail to find it... since every source I trace down seems to define the term a bit different. For example: some even put AES into the basket when they were able to make it run on some limited-abilities piece of hardware, while others explicitly exclude AES from the lightweight crypto group. Same for ChaCha20, etc.
To avoid confusion, knowing the official definition (and maybe even having a reference to point to) would definitely help.
TL;DR
What is the official, generally accepted definition of &quot;lightweight crypto&quot;? And where (reference) is it defined?
There is no official definition, of course. However, at least in my experience, the word is used consistently.

The word lightweight typically refers to something that is significantly less expensive to use than other stuff, while at the same time achieving the same desired effect, but this lightweight-ness has some cost. (If there was no cost to lightweight-ness, it would not be lightweight, but just "better".) This means that the lightweight stuff may be desirable in some situations, but not in others.

Typically, a lightweight block cipher will be amenable to implementations that are small or use little power or ..., compared to the usual block ciphers. This happy state is achieved at some cost, perhaps by reducing the security margin or having more rounds, etc.

Note that people may talk about lightweight public key cryptography, but it is lightweight only when compared to ordinary public key cryptography, not compared to other kinds of cryptography such as block ciphers.

People also talk about lightweight implementations of cryptographic primitives. Again, lightweight refers to some reduced cost, typically achieved by increasing some other cost. People may be interested in an AES implementation with extremely small gate count, for instance, even if it would be slower or use more power than other implementations.

The word lightweight is of course not limited to cryptography, but is used everywhere, with somewhat differing meanings. Consult your dictionary. In technical contexts, it usually has positive connotations in my experience.