From the cited RFC,

If double-quotes are used to enclose fields, then a double-quote appearing inside a field must be escaped by preceding it with another double quote. For example:

"aaa","b""bb","ccc"

and

...from the preceeding para of the RFC ( http://tools.ietf.org/html/rfc4180#section-2 ):

If fields are not enclosed with double quotes, then double quotes may not appear inside the fields.

Note, however that "as defined" may overstate the status of the document:

This memo provides information for the Internet community. It does not specify an Internet standard of any kind.
... While there are various specifications and implementations for the CSV format (cites removed), there is no formal specification in existence, which allows for a wide variety of interpretations of CSV files. This section documents the format that seems to be followed by most implementations: