ESCROWED SHARES

Shares held in an escrow account and in most cases cannot be traded or transfered until certain circumstances like time horizon have been reached. The use of escrow for holding shares is often done during acquisitions and for performance-based executive incentives.

These shares can be held by an escrow company or by the exchange the shares trade on.
Let's say a company puts up shares as a guarantee on an acquisition. Should that firm rescind the offer, they're likely to lose the shares.