obligatory
/ɒˈblɪgətərɪ/adjective
1- required by a legal, moral, or other rule; compulsory（根据法律、道德标准或其他规章的规定）应承担义务的, 必须履行的; 强制性的：
use of seat belts in cars is now obligatory.
如今在车上系安全带是强制性的规定。
1.1
- often humorous so customary or fashionable as to be expected of everyone or on every occasion〈常幽默〉（依照习惯或风尚）少不了的; 必有的：
after the obligatory preamble on the weather he got down to business.
在习惯性的关于天气的开场白之后他开始了正题。
1.2
- (of a ruling) having binding force（规定, 裁决）有约束力的：
a sovereign whose laws are obligatory.
其法律有约束力的主权国家。
派生词
obligatorily
adverb词源
late Middle English: from late Latin obligatorius, from Latin obligat- 'obliged', from the verb obligare （see OBLIGE）.