If you mean a false statement:
* Lie (this is the noun form of the verb "to lie")
* Untruth
* Fabrication
* Deception
* Falsehood
* Fib (informal)
If you mean the act of lying down:
* Reclining
* Rest
* Position
* Prone position
Let me know if you have a specific situation in mind, and I can give you a more accurate answer!