EDIT: I didn't mean to get so philosophical about this, I was really just looking for a general definition. You are correct, Sarkus, that I'm not pondering what it means to exist, but rather "when" things exist (only because present tense of the word requires that we refer to a item/event in spacetime).
You are in the Philosophy section, and yes, it is a highly philosophical question, no matter how you want to look at it.
Exit:
To have life, and being, to be real, anything that can be touched, seen, smelt, inferred by its effects on other things......all in all, common knowledge I would think.
Here are a few words of wisdom regarding Philosophy:
Science is what you know. Philosophy is what you don't know.
Bertrand Russell (1872-1970) English philosopher, mathematician.
There is no statement so absurd that no philosopher will make it.
Cicero, Marcus Tullius (106-43 BCE) Roman statesman. De Divinatione
Philosophy, n. A route of many roads leading from nowhere to nothing.
Ambrose Bierce (1842-1914?) American writer. (The Devil's Dictionary, 1911)
Metaphysics is a dark ocean without shores or lighthouse, strewn with many a philosophic wreck.
Immanuel Kant (1724-1804) German Philosopher
Philosophy of science is about as useful to scientists as ornithology is to birds.
Attributed to Richard Feynman (1918-88) U.S. Physicist. Nobel Prize 1965.
...philosophy is to science as pornography is to sex.
Steve Jones
Philosophy consists very largely of one philosopher arguing that all others are jackasses. He usually proves it, and I should add that he also usually proves that he is one himself.
Henry Louis Mencken. (1880-1956). Minority Report, H. L. Mencken's Notebooks. Knopf, 1956.
Scientists are explorers. Philosophers are tourists.
Richard Feynman
http://www.lhup.edu/~dsimanek/sciquote.htm