What does perfect mean within perfect forward secrecy and why do some cryptographers disagree with the term?
Forward secrecy of key-establishment (KE) protocol means, that compromising of long-term keys of parties doesn't affect secrecy of established keys in the past.
Example to clarify what PFS means.
Say parties A and B have private keys $s_B$ and $s_B$.
Today they run KE and establish a key $k_1$.
They use this key during a session, and then destroy this key.
During this session, attacker eavesdropped all the messages and remembers the transcript $T$ of the session. Obviously, $T$ includes messages of KE,
and subsequent &quot;data&quot; messages, encrypted with key $k_1$.
Tomorrow, attacker somehow gets keys $s_A$ and $s_B$, but not $k_1$ (because $k_1$ was destroyed and forgotten by $A$ and $B$).
And here comes PFS property of KE: even having $T$, $s_A$ and $s_B$, attacker is unable to find $k_1$ and decrypt messages hidden in $T$.
Then answering your question, why in PFS word &quot;perfect&quot; is ambiguous.
It's an opinion of M.Green obviously, but I will try to guess what he means.
Actually I feel pretty the same regarding this term. &quot;Perfect&quot; is ambiguous here.
E.g., we don't use &quot;perfectly secure signature scheme&quot;, we just use &quot;secure&quot;.
Moreover, in cryptography word &quot;perfect&quot; appears sometimes in order to describe
that some property (usually some sort of secrecy) is based not on computational assumptions, but it's perfect - i.e., unconditional and doesn't require any assumptions/hypotheses.
Examples:

perfect zero-knowledge vs zero-knowledge (which could be just statistical or computational)
Perfect secrecy of encryption - term introduced by Shannon https://en.wikipedia.org/wiki/One-time_pad#Perfect_secrecy

So, in cryptography &quot;perfect&quot; is usually a synonym to
https://en.wikipedia.org/wiki/Information-theoretic_security.
But for some stupid reason - not in this case (in case of KE).
So, more clear would be to have a term &quot;Forward Secrecy&quot;, and then, additional &quot;Perfect&quot; would mean that it doesn't require any assumptions and it's unconditional.
But for some historic reason, in literature PFS is used for regular forward secrecy.