Hard-To-Borrow List

What Does Hard-To-Borrow List Mean?
A list used by brokerages to indicate securities considered difficult or unavailable to borrow for short selling transactions.

Investopedia explains Hard-To-Borrow List
The hard-to-borrow list expedites short selling transactions. According to the SEC and NASD, securities not found on this list are assumed to be in sufficient quantity and available for short sales. Those stocks on the list are still available for short selling - however, brokerages and investors must take extra measures to search the stocks out and ensure their delivery before they are sold short.