Rambam - Sefer HaMitzvos: Negative Mitzvah 62: Negative Mitzvah 62

Negative Mitzvah 62: We are prohibited from taking vain oaths
Exodus 20:7 "You shall not take the name of the L-rd, your G-d in vain"

This Negative Mitzvah teaches not to make a "Shevuat Shav", which means a "vain promise."

A Shevuat Shav can be one of three different types of promises:

To swear that something is not what it really is.
For instance: When Sarah got her new bicycle she told her friends that it cost $10,000. If Sarah would have sworn that her bicycle costs $10,000, when it did not - this would have been a Shevuat Shav.
To swear that something exists, when such a thing is impossible.
For instance: After Danny went to the zoo with his parents, he told his friends that in the zoo he saw an elephant with wings and the elephant flew through the air. If Danny would have sworn that he had seen such an elephant, which is impossible, he would have made a Shevuat Shav.
To swear about something which is plainly obvious.
For instance: To swear that a cow is a cow.
To swear that you will not keep one of the Mitzvot in the Torah.
For instance: to swear that you will not say Shema today is considered a Shevuat Shav.

All these promises are part of the prohibition of Shevuat Shav, and neither they, nor anything like them, should be made - even if they are made in a joking manner!

There are times when everything about you, the entire world in all its chutzpah, denies the truth you know within. There are times you must be a lion, a deer, an eagle, a tree -- but now you must be a rock.
Now you must not flinch, not in any way even acknowledge the existence of the mighty waves that come crashing down upon you, conspiring to grind you to sand, to sweep you away to join them in the vast ocean. You must be the hard, unmoving rock that lies at the essence of your soul, the voice from beyond all this ephemeral reality, from beyond all time and space, that says, "They are nothing. There is none else but He." It begins with you. And then it happens in your world: The outer crust of facade begins to crack, the essential reality is revealed, the storm dissipates as though it never were, and all things begin to say, "I am not a thing. In truth, there is none else but G-d."

From: Bringing Heaven Down to Earth by Tzvi Freeman - tzvif@aol.com