Definition of Noise Margin?

http://en.wikipedia.org/wiki/Noise_margin

i don't know the exact answer.from my view it is the external thing that is added to the input which does not cause any undesirable changes at the output........

That's a nice simple definition...

I was going to ask 'In what context' ...but your reply works independant of context.

I doff my cattly hat!

its a difficult problem. different system has different noise margin.

however, if there is esisting a very little you adoptable, recievable, comprehensvable informaiton means the least margin of noise.

In digital logic, signal presence is marked as 5v dc ( or logic "1"). Same way no signal is marked as zero voltage ( or logic "0"). All logic circuits accepts logic 1 if voltage >3.5v DC. logic zero if voltage <2.5vDc. so voltage level between 2.5 to 3.5 is called noise margin. In this region ,we can't say signal is logic 1 or 0. Here voltage level < 2.5v and > 3.5v is explanation only not exactly correct, you refer your data sheet of particular IC.

Im afraid your example is not very right.( sorry for my comment)

generally speaking noise signal is not equal to undetermination zone signal.