Search notes:

/var/lib/portage/world

/var/lib/portage/world contains the list of packages that were explicitly installed by a system administrator.
Before installing app-editors/vim, the package is not found in the world file: 
# grep vim /var/lib/portage/world
After installing this package, it is recorded in the world file: 
# emerge --ask app-editors/vim
…
# grep vim /var/lib/portage/world
app-editors/vim

See also

Using the --oneshot argument of emerge avoids adding a package to the world file.

Index