Difference between acceptance test and functional test?

Question

What is the real difference between acceptance tests and functional tests?

What are the highlights or aims of each? Everywhere I read they are ambiguously similar.

Answer 1

I like the answer of Patrick Cuff. What I like to add is the distinction between a test level and a test type which was for me an eye opener.

test levels

Test level is easy to explain using V-model, an example: Each test level has its corresponding development level. It has a typical time characteristic, they're executed at certain phase in the development life cycle.

1. component/unit testing => verifying detailed design
2. component/unit integration testing => verifying global design
3. system testing => verifying system requirements
4. system integration testing => verifying system requirements
5. acceptance testing => validating user requirements

test types

A test type is a characteristics, it focuses on a specific test objective. Test types emphasize your quality aspects, also known as technical or non-functional aspects. Test types can be executed at any test level. I like to use as test types the quality characteristics mentioned in ISO/IEC 25010:2011.

1. functional testing
2. reliability testing
3. performance testing
4. operability testing
5. security testing
6. compatibility testing
7. maintainability testing
8. transferability testing

To make it complete. There's also something called regression testing. This an extra classification next to test level and test type. A regression test is a test you want to repeat because it touches something critical in your product. It's in fact a subset of tests you defined for each test level. If a there's a small bug fix in your product, one doesn't always have the time to repeat all tests. Regression testing is an answer to that.