Compulsory, mandatory, and obligatory

Adriano from Brazil asked: Is there any difference between compulsory, mandatory, and obligatory?

These three adjectives are confusing  because the main definition given in the dictionary for all three of these words is the same. Compulsory, mandatory, and obligatory can all mean “required by a law or a rule,” as shown in these example sentences:

1. Massachusetts was the first state to pass a compulsory school attendance law. (=a law that requires everyone to attend school.)
2. The company initiated mandatory drug testing for all employees. (=drug testing required for all employees)
3. The meeting is obligatory for all employees. (=all employees must attend)

The difference between these words is in how commonly they are used, and in what contexts.

The word mandatory is the most common of the three choices, and it is the least formal. It is particularly common in the phrases mandatory testing, mandatory sentencing and mandatory retirement. 

Compulsory is less common.  It is most often found in the phrases compulsory military service and compulsory education.

Obligatory is the least common of these three words, and the most formal. It is rarely used in spoken language. Obligatory also has a second meaning that is quite different from the first. It can describe something that is not required, but has become so common or typical that it now seems overused and not very meaningful or effective, as in this example sentence: 

• This action movie includes the obligatory chase scene.

In general, if you want to describe something that is required by law or by a rule, and you're not sure which adjective to use, mandatory is a good choice, because it is the most common. 

I hope this helps.