Definition
This phrase is used to indicate that an action must be taken immediately, especially because there will not be another opportunity.
The company is offering a limited-time discount; if you don't buy now, you'll miss out—it's really 'now or never'.
If you want to apply for this position, you must submit your application now; it’s 'now or never'.
They must make a decision today because the contract will expire after today; it's 'now or never'.