Attack

ISTQB Glossary  definition 

Attack is directed and focused attempt to evaluate the quality, especially reliability, of a test object by empting to force specific failures to occur.

See also negative testing.

Note: For some reason this is not listed as part of the new version 3.01 of the glossary. I guess this is a an oversight and probably will be rectified in the future releases. how ever in the sprite of learning and knowing your trade, I have included it here.

In Simple English,

A much easier way to understand Attack is to think of it as testing for defects that were successfully fixed in previous releases.

You know or assume that a particular part of the application under test will break or
you do not want a particular feature or part of the application under test to break so you very specifically test that particular feature or previous failure. This is termed as an attack.

Field Notes 

• Attack as a term is used mostly as part of security testing jargon.  They really have attacks to simulate and test the capacity of the system to withstand the attacks.
• Not many use this term for functional testing as much as i have seen.
• Though this is very close to negative testing, do not confuse this with negative testing as they are both different things.
• A good way to understand this would be to see an Attack as an very specific method of testing which could also be grouped under the negative testing category. 

For Example:
I had to think a lot about a better example for this other than a user id and password based example scenario just so you understand the actual flavour of this test technique.

still thinking.., if you have any good examples .., please leave them in the comments below.