Avak Lashon Hara is a Rabbinic prohibition mentioned in the gemara and detailed in the Rambam (Hilchot De’ot 7:4) and in Rabeinu Yonah’s Shaarei Teshuvah (3:126). Avak Lashon Hara is defined as words which have an inference of gossip, even though they are not as clearly negative as gossip itself.
In more detail, if a person implies something negative about another, rather than directly relating it, it will be considered avak lashon hara. (Whereas if he were to spell it out clearly and directly it would be the Torah-prohibited gossip.)
Examples:
“Who could ever believe that Reuven would be appointed to such a position?!” This implies that Reuven is not worthy of the position because of a certain failing.
“Don’t ask me about Shimon, I don’t want to speak about him,” which implies that there is something negative about Shimon.
“At least Levi is not lazy,” which is a praise of Levi designed to imply that he has other failings.
Another common form of avak lashon hara involves merely mentioning a negative aspect of another by the way, as if the speaker is not intending any harm by saying it. Such a thing could end up damaging the victim, but the speaker would deny that he had spoken gossip.
Someone who praises Yehuda in the presence of those who hate Yehuda is guilty of avak lashon hara, because those who hate him will surely pipe up with negative comments about him, trying to counterbalance the praises. (See Arachin 16a) One should not praise someone excessively even in the presence of others who are not known to hate that other, since it is likely that those present will suggest that the speaker exaggerated and try to point out his faults. The exception to this rule is when talking about a genuine, well-known complete Tzaddik – then there is no reason to fear that someone will try to criticize such a saintly person.
If a person relates something negative about another without mentioning his name, if it is clear from the details that he shares about whom he is speaking, it is also considered avak lashon hara. Some opinions consider this full gossip (Torah-prohibited). Therefore, one must be extremely careful not to fall into this trap.