Effective means it actually works — it does the job it was meant to do. A cheap rubber doorstop that holds a heavy door is effective; an expensive one that slides across the floor is not.
The pair to keep straight is effective and efficient: effective is about the result, efficient is about how little time, money or effort it costs. A bucket of water is effective on a small fire; a fire extinguisher is efficient. Effective also means in force, the way a rule or a price starts on a date: the new fares are effective from 1 May. And it can mean real rather than official — she was the effective head of the department for two years without the title. The adverb effectively carries both senses, so "he effectively resigned" means that in practice he did. The nouns are effect and effectiveness.
1adjectiveeffectiveProducing the result it was meant to produce.
“Cold water is surprisingly effective on a fresh stain.”
synonyms: successful, powerful
2adjectiveforceIn force; applying from a stated date.
“The pay rise is effective from the first of April.”
3adjectiverealReal in practice, though not official.
“The deputy was the effective manager for most of that year.”
Too few real examples of this word to estimate which sense is commonest.
Effective came from French effectif, from Latin effectīvus, "productive, effective", built on efficiō, "to make, to bring about". English could equally have assembled it at home out of effect plus -ive. Both routes land in the same place: the word is about what actually gets brought about.
French
effectif
English, today
effective
Built from
From French effectif. From French effectif, from Latin effectīvus (“productive; effective”), from efficiō (“to make; to bring about”), equivalent to effect + -ive.
Etymology from Wiktionary, CC BY-SA