Effective

1. Having the power to produce a required effect or effects. Synonym: efficacious
2. Producing a decided or decisive effect.
3. Efficient, serviceable, or operative, available for useful work.
4. Actually in effect.
5. (geometry, of a cycle or divisor) Having no negative coefficients.
6. (physics, for any effective theory) approximate; Not describing the fundamental dynamic changes in some system as they happen.

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