What does "inet" stand for in the ip utility?

inet = Internet protocol family

inet6 = Internet protocol version 6 family

manpage inet

DESCRIPTION

The Internet protocol family is a collection of protocols layered atop the Internet Protocol (IP) transport layer, and utilizing the Internet address format. The Internet family provides protocol support for the SOCK_STREAM, SOCK_DGRAM, and SOCK_RAW socket types; the SOCK_RAW interface provides access to the IP protocol.

manpage inet6

DESCRIPTION

The inet6 family is an updated version of inet(4) family. While inet(4) implements Internet Protocol version 4, inet6 implements Internet Protocol version 6.

inet6 is a collection of protocols layered atop the Internet Protocol version 6 (IPv6) transport layer, and utilizing the IPv6 address format. The inet6 family provides protocol support for the SOCK_STREAM, SOCK_DGRAM, and SOCK_RAW socket types; the SOCK_RAW interface provides access to the IPv6 protocol.

The inet in ifconfig output is an address family:

If the first argument after the interface name is recognized as the
name of a supported address family, that address family is used for
decoding and displaying all protocol addresses.  Currently supported
address families include inet (TCP/IP, default), inet6 (IPv6), ax25
(AMPR Packet Radio), ddp (Appletalk Phase 2), ipx (Novell IPX) and
netrom (AMPR Packet radio).

man 7 address_families leads to man 7 ip, which mentions netinet/in.h (POSIX), which in turn mentions arpa/inet.h (POSIX).

And the description of arpa/inet.h is:

arpa/inet.h - definitions for internet operations

So, pretty much, "internet."