During the Ordination of a bishop, the recipient receives a ring, pallium, and crosier (staff). Each is explained during the Rite:

Receive this ring, the seal of fidelity: and adorned with undefiled faith, preserve unblemished the Bride of God, the holy Church.  

Receive the pallium, taken from the tomb of blessed Peter, which we hand on to you in the name of the Roman Pontiff, Pope Francis, to wear within the bounds of your ecclesiastical province as a sign of the authority of a Metropolitan; let it be for you a symbol of unity, a distinctive sign of communion with the Apostolic See, a bond of charity, and a spur to courage.

Receive the crosier, the sign of the pastoral office, and keep watch over the whole flock in which the Holy Spirit has placed you as Bishop to govern the Church of God.

Those three things are a seal, sign, symbol, bond, or spur.

The recipient also receives the bishop’s miter (headdress). Here’s the explanation:

Receive the miter, and let the splendor of holiness shine in you, so that when the chief Shepherd appears you may merit to receive an unfading crown of glory.

Is the bishop’s miter not a seal, sign, symbol, bond, or spur? What is its significance? The search for the answer may start with one word in the explanation. Splendor is “Great brightness; brilliant light or luster. (OED)” Maybe the Church prays during the rite that God transforms the miter’s luster into the visible, shining quality of the bishop’s invisible, radiant holiness.