A licence that gives you the right to own and use a vehicle in Singapore for 10 years, bid for in twice-monthly LTA bidding exercises. Because the number of COEs is capped to control the vehicle population, the price is set by supply and demand and can swing widely.
Example: A Category A car COE (for smaller, less powerful cars) can cost tens of thousands of dollars on top of the car's base price, and the quota premium is non-refundable once the 10 years are up.
After 10 years you must either deregister and dispose of the vehicle, or renew the COE by paying the Prevailing Quota Premium (a moving average of recent COE prices) for another 5 or 10 years. A renewed COE has no rebate value at the end.
The COE is a quota system: the government caps how many vehicles can be on the road, so buyers bid against each other for a limited number of certificates. When demand is high relative to the quota, premiums rise. Prices change at each bidding exercise and should be checked against LTA's latest results.