System-wide environment variable in Ubuntu

/etc/environment
/etc/profile
/etc/profile.d/*

The Ubuntu wiki suggest not to modify /etc/profile, but adding files in /etc/profile.d because /etc/profile will source the file in /etc/profile.d.

The setting specified in /etc/profile will take into effect while a user logged in.

Ref: https://help.ubuntu.com/community/EnvironmentVariables#A.2BAC8-etc.2BAC8-environment