According to the language specification:
Identifier Names are tokens that are interpreted according to the grammar given in the “Identifiers” section of chapter 5 of the Unicode standard, with some small modifications. An Identifier is an IdentifierName that is not a ReservedWord (see 7.6.1).
This means you cannot use delete as a variable or function name, but you can use it as an object property name. This was not the case in the previous version of the specification, which is why most other answers are recommending you avoid using reserved words entirely. However, in ES5-compliant implementations, there should be no problem.