Sell Wall / Buy Wall
A large cluster of orders at one price in the order book — real support/resistance, or a bluff meant to be seen.
A sell wall is an unusually large sell order (or cluster) sitting at one price in the order book, appearing as a tall step on the depth chart; a buy wall is the same on the bid side. They can act as strong short-term resistance or support, since price must consume the whole wall to move through it.
But walls are often theater. A trader may place a large wall to create the impression of support or resistance — influencing others — then pull it before it fills (a form of spoofing). So a visible wall is as likely to be a psychological bluff as a genuine commitment. Reading walls means always asking whether the order will actually be there when price arrives.