Lag means to fall behind or move more slowly than others. When you're hiking with friends and start to lag behind, you're not keeping up with the group's pace. In a race, the runner who lags is trailing the leaders.

The word often describes a delay between when something should happen and when it actually does. When you're playing an online video game and experience lag, there's a frustrating pause between when you press a button and when your character responds on screen. This happens because data takes time to travel across the internet to the game's servers and back.

Scientists and economists use lag to describe delays in how systems respond. When a thermostat adjusts the temperature, there's a lag before the room actually gets warmer. When the government changes economic policies, there's often a lag of months before the effects show up.

Jet lag is the tired, confused feeling you get after flying across time zones, when your body's internal clock lags behind the actual time at your destination. The related word laggard describes someone who habitually falls behind or moves slowly, though it sounds a bit old-fashioned today.