Numerics
Numerics are the name of the commands that are three digits long. They range from '000' to '999' and cannot be sent from a client to a server.
Numerics have names such as RPL_WELCOME, RPL_ISUPPORT, and RPL_ENDOFMOTD. For example, the numeric '001' means RPL_WELCOME, and the numeric '306' means RPL_AWAY.
These names and the meaning of different numerics are defined in RFC1459 and RFC2812. However, many others have been defined and are used by IRC networks today which are not defined in these specifications (such as '005' which means RPL_ISUPPORT). Some numerics defined in these specifications now mean something different. For instance, the numeric '005' is defined as RPL_BOUNCE in RFC2812, when it actually means RPL_ISUPPORT when used by networks today.
An extensive list of numerics, including obsolete and conflicting definitions, can be found on the irc-defs numerics list page.