Efficient

1. making good, thorough, or careful use of resources; not consuming extra. Especially, making good use of time or energy
2. expressing the proportion of consumed energy that was successfully used in a process; the ratio of useful output to total input
3. causing effects, producing results; bringing into being; initiating change (rare except in philosophical and legal expression efficient cause = causative factor or agent)
4. (proscribed, old use) effective

Usage notes

Webster's Revised Unabridged Dictionary from 1913 still lists efficient and effective as synonyms, but all major dictionaries now show that these words now only have different meanings in careful use. Use of both for the other meaning is however widespread enough that Longman's Exam Dictionary, for example, finds it necessary to proscribe the use of one for the other with several examples at each entry and provides the following summary:

• efficient (“working quickly and without waste”)
• effective (“having the desired effect”)

Antonyms

• inefficient