I'm looking for a specific word, but I can't remember what it was. It begins with "a" I believe, and its definition is something along the lines of "successful in its endeavor, successful in its purpose". It's an adjective, and you would use it to describe something that was or would be successful in it's goal. For example, you could describe a product that did as it advertised using this adjective. I believe it's 3 or 4 syllables but I might be wrong.
EDIT: I was wrong. It began with "e". efficacious was the word I was looking for.