No one has a very precise definition of liquidity, but it roughly boils down to how easy it is to buy or sell a particular security and how much agreement there is in the marketplace upon the security's fair value. The most liquid funds or stocks have miniscule bid-ask spreads, where the prices differ by only a penny. On the other side, a brand new ETF tracking a selection of more thinly traded mortgage-backed securities has a bid-ask spread near 0.80% as I write this. That means that buying and selling the fund at market prices, even without any commissions charges or price changes, would result in a 0.80% loss. Not exactly a terrifying loss, especially compared with what we all saw in 2008, but still an unwelcome drag on portfolio returns if it can be avoided.
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