"Sub" (visual definition)

Cf. verbal definitions e.g. "a prefix, occurring orig. in loanwords from Latin, with the meanings “under,” “below,” “beneath” (subsoil; subway), “just outside of,” “near” (subalpine; subtropical), “less than,” “not quite” (subhuman; suboscine; subteen), “secondary,” “at a lower point in a hierarchy” (subcommittee; subplot)."

https://www.thefreedictionary.com/Sub