If on, writes fail unless you use a '!'. Protects you from accidentally overwriting a file. Default on when Vim is started in read-only mode ("vim -R") or when the executable is called "view". When using ":w!" the 'readonly' option is reset for the current buffer, unless the 'Z' flag is in 'cpoptions'. When using the ":view" command the 'readonly' option is set for the newly edited buffer. See 'modifiable' for disallowing changes to the buffer.