Liquidity means the ability to transact in size without moving the price much. It sounds like a technical detail. It is the property that determines whether a market functions.
Measuring it
Several measures exist and none is complete.
The spread between the best bid and the best offer is the most visible, and it captures only the cost of a small transaction.
Depth — the volume available at and near the best prices — matters for anything larger.
Resilience — how quickly depth is replenished after a trade consumes it — matters most and is hardest to observe.
A market can show a tight spread and almost no depth, which looks liquid and is not.
Who provides it
Market makers quote both sides continuously, earning the spread and carrying inventory risk.
Their willingness to do this depends on the risk of being on the wrong side of an informed trade.
When uncertainty rises, that risk rises, and the rational response is to widen quotes and reduce size.
Which means liquidity contracts precisely when it is most needed, and this is not a market failure but the predictable result of the incentives.
The obligation question
Designated market makers on some venues have formal obligations to quote, with specified maximum spreads and minimum sizes.
Much modern liquidity provision carries no such obligation and can withdraw entirely.
The shift from obligated to voluntary provision is one of the structural changes in market making over recent decades, and its consequences appear during stress.
Flash events
Episodes where prices move violently and recover within minutes have occurred in equities, currencies and government bonds.
The common pattern is a liquidity withdrawal rather than a change in fundamental information.
An order arrives, available depth is consumed, the price gaps to wherever the next resting order sits, and automated systems widen or withdraw in response, deepening the gap.
Then it reverses as participants return.
Circuit breakers and volatility auctions were introduced substantially in response, pausing trading to allow depth to reassemble.
The fund liquidity mismatch
A more consequential version of the same problem.
A fund offering daily redemption while holding assets that take weeks to sell has a structural mismatch.
In normal conditions this is invisible, because redemptions are modest and can be met from cash.
Under stress, redemptions rise, and meeting them requires selling the most liquid holdings first, which leaves the remaining holders with a less liquid portfolio.
Which creates an incentive to redeem early, and that is a run.
Several property funds and credit funds have suspended redemptions for exactly this reason, and each time the structure was known in advance to be vulnerable.
Tools that address it
Swing pricing adjusts the price at which redemptions are met to pass transaction costs to redeeming holders rather than to those remaining.
Redemption gates limit the proportion that can be withdrawn in a period.
Notice periods align the redemption terms with the underlying asset liquidity, which addresses the mismatch directly and is commercially unattractive to sell.
Regulators have pushed toward the first two and, in some markets, toward the third.
Government bond markets
Worth a specific mention because they are assumed to be the most liquid market and have shown strain.
Dealer balance sheet capacity, constrained by post-crisis capital rules, has not grown alongside the outstanding stock of government debt.
Which means intermediation capacity relative to market size has fallen, and episodes of disorder in supposedly the safest market have occurred.
Central bank intervention has been required more than once, which is itself informative about the underlying capacity.
The practical implication
For a long-term holder of broadly traded assets, liquidity is mostly somebody else's problem, and it becomes yours only if you need to sell at the wrong moment.
Which is the actual argument for holding cash reserves separately — not because cash performs well, but because it removes the need to transact in illiquid conditions.
That is a structural observation rather than advice, and individual circumstances determine what follows from it.
Market structure fragmentation
A change that affects how liquidity should be measured.
Trading in a single security occurs across multiple venues — primary exchanges, alternative venues and private matching systems.
Which means depth on any one venue understates the total available, and a consolidated view is required to assess it properly.
Consolidated data exists in some markets and not others, and where it does not, participants without expensive infrastructure genuinely cannot see the whole picture.
Regulatory work on consolidated reporting has been ongoing for years in several jurisdictions, with slow progress against resistance from venues whose data is commercially valuable.
The exchange traded fund layer
Worth mentioning since it is frequently misunderstood.
An exchange traded fund's liquidity is not limited to its own trading volume, because authorised participants can create and redeem units against the underlying holdings.
Which means a thinly traded fund holding liquid assets is more liquid than its volume suggests.
The reverse is the concern — a heavily traded fund holding illiquid assets, where the fund's apparent liquidity exceeds what its holdings can support under stress.