Optional vs. Obligatory — What's the Difference?

Left to choice; not compulsory or automatic.

Required by a legal, moral, or other rule; compulsory

Not compulsory; left to personal choice; elective.

Of the nature of an obligation; compulsory

Something that is not compulsory, especially part of an academic course.

Required as part of a routine course of action or cultural expectation; predictable

In various programming languages, a kind of variable that is assigned a specific data type but may or may not hold an actual value.

(Biology) Obligate.

Involving an option; depending on the exercise of an option; left to one's discretion or choice; allowed but not compulsory; as, optional studies; it is optional with you to go or stay.

Imposing obligation, legally, morally, or otherwise; binding; mandatory.

Possible but not necessary; left to personal choice

Requiring a matter or obligation.

Binding in law or conscience; imposing duty or obligation; requiring performance or forbearance of some act; - often followed by on or upon; as, obedience is obligatory on a soldier.

Morally or legally constraining or binding;

Required by obligation or compulsion or convention;