A lie is when there is deception involved, as dashingScorpio said. That includes withholding knowledge or allowing knowledge you know to be false to be propagated. That's usually referred to as "lying by omission" because you are not verbally lying, In the other cases you mentioned, such as trying to appear more prosperous, I think everyone tries to make themselves look better as part of human nature and ego, but it could also be classified as "false advertising" though I'm not sure I would personally consider it lying. If you are flirting with someone who you have no interest in, or you are not available due to another relationship, I would classify it as both lying and false advertising, as well as just mean. I'm not sure anyone needs a specific definition of a lie, because it is different for everyone and furthermore, most people know when they are lying, and do so with intent, and finally lots of people know or will find out that they have been lied to, which defeats the purpose of the lie in the first place and only adds more drama to our lives.