Calm, peaceful (6)
(Other definitions for placid that I've seen before include "Calm; equable" , "Peaceful and tranquil" , "Peaceful - composed" , "Peaceable and not easily disturbed" , "Calm, peaceful" .)
I believe this is a double definition.
'calm' is the first definition.
(I know that calm can be written as serene)
'peaceful' is the second definition.
(similar in meaning)
(Other definitions for serene that I've seen before include "Peaceful and calm in manner" , "Calm and unruffled, like a princely title." , "Calm, peaceful, tranquil" , "Calm and unclouded" , "Calm and relaxed" .)