The user configuration file for the tmux terminal multiplexer, .tmux.conf, supports defining and using environment variables in the configuration, with the same syntax as most shell script languages:

TERM=screen-256color
set-option -g default-terminal $TERM

This can be useful for any case in which it may be desirable to customise the shell environment when inside tmux, beyond setting variables like default-terminal. However, if you repeat yourself in places in your configuration file, it can also be handy to use them as named constants. An example could be establishing colour schemes:

TMUX_COLOUR_BORDER="colour237"
TMUX_COLOUR_ACTIVE="colour231"
TMUX_COLOUR_INACTIVE="colour16"

set-window-option -g window-status-activity-bg $TMUX_COLOUR_BORDER
set-window-option -g window-status-activity-fg $TMUX_COLOUR_ACTIVE
set-window-option -g window-status-current-format "#[fg=$TMUX_COLOUR_ACTIVE]#I:#W#F"
set-window-option -g window-status-format "#[fg=$TMUX_COLOUR_INACTIVE]#I:#W#F"

The explicit commands to work with environment variables in .tmux.conf are update-environment, set-environment, and show-environment, and are featured in the manual.