(typically of something inanimate or abstract) successful in producing a desired or intended result; effective
(尤指无生命或抽象事物)有效的,奏效的
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tobacco smoke is the most effectual protection against the midge .烟草的烟雾是最有效的防蚊剂。
【律】 (法律文件)有法律效力的,有约束力的。
"late Middle English: from medieval Latin