Gold, foreign exchange reserves, and short-term government bonds from reliable issuers are traditionally considered defensive. They are unified not by high returns, but by relative stability when risky assets decline in value.
The mechanism varies. Short-term government securities are stable because little time remains until maturity and revaluation is minimal. Gold is not tied to any issuer's obligations and historically sought after when confidence in financial instruments wanes. Foreign currency hedges against national currency weakness but does not protect against inflation in that currency itself.
The price of stability is low returns during calm periods. A portfolio composed entirely of defensive assets underperforms inflation over the long term, so their role is as a portion rather than a complete portfolio replacement.
Important caveat: defensive properties do not exist in absolute terms. Gold can be volatile, and the "reliability" of government securities depends on the issuer and currency of issue. For an investor with expenses in a specific currency, the defensive function is determined by how the asset performs relative to their own future spending needs, not relative to global markets.