2.adjectiveMaking good, thorough, or careful use of resources; not consuming extra. Especially, making good use of time or energy
"An efficient process would automate all the routine work."
3.adjectiveExpressing the proportion of consumed energy that was successfully used in a process; the ratio of useful output to total input
"The motor is only 20% efficient at that temperature."
4.adjectiveCausing effects, producing results; bringing into being; initiating change (rare except in philosophical and legal expression efficient cause = causative factor or agent)
"Ownership, maintenance, or use of the automobile need not be the direct and efficient cause of the injury sustained"