Variable (9)
(Other definitions for parameter that I've seen before include "Limitation" , "Factor defining limits" , "Limiting factor" , "Quantifiable characteristic of a system" , "One of a set of factors as conditions for operation of a system" .)
'variable' is the definition.
(similar in meaning)
This is all the clue.
(Another definition for changeful that I've seen is " Fluid".)