Absence

Absence is the opposite of presence.

The terms absence and presence deserve technical definition due to their prominence in phenomenology.

"opposite" also deserves technical definition since "opposition" is also a term in metaphysics. For example, "ontology is the opposite of phenomenology" is a technically meaningful (if possibly wrong) sentence that can be definitional -- even canonical -- so long as "opposite" and "phenomenology" are properly defined. It may, perhaps, be more accurate to say "ontology is the dual of phenomenology" as canonical so long as one can then adequately define "dual" and "phenomenology".