Liquidity
How easily you can enter/exit without moving price. Deep = tight spreads and fast fills. Thin = slippage and gaps.
Majors during London or NY have deep liquidity. Asian session on crosses is thin. Bank holidays kill liquidity anywhere. In our system, 'liquidity' also refers to resting stop orders (BSL/SSL) that institutions sweep.
Related
Learn to actually use Liquidity.
Definitions are the easy part. The free first five modules put this on a real chart and make you do the work. No card required.