1st link: Dark pools of liquidity (also dark pools or dark liquidity) are crossing networks that provide liquidity that is not displayed on order books. This is useful for traders who wish to move large numbers of shares without revealing themselves to the open market.

Dark liquidity pools offer institutional investors many of the efficiencies associated with trading on the exchanges' public limit order books but without showing their hands to others. Dark liquidity pools avoid this risk because neither the price nor the identity of the trading company is displayed.


2nd link:What Does Dark Pool Liquidity Mean?
A slang term that refers to the trading volume created from institutional orders, which are unavailable to the public. The bulk of dark pool liquidity is represented by block trades facilitated away from the central exchanges.

Also referred to as the "upstairs market", or "dark liquidity", or just "dark pool."
Investopedia Says
Investopedia explains Dark Pool Liquidity
The dark pool gets its name because details of these trades are concealed from the public, clouding the transactions like murky water. Some traders that use a strategy based on liquidity feel that dark pool liquidity should be publicized.