Front Month
The nearest expiration month for an options contract.
Explanation
Front-month options have the highest theta decay rate and are most sensitive to short-term price movements. They are the most actively traded and typically have the tightest bid-ask spreads. Calendar spreads exploit the difference in time decay between front-month and back-month options.
Example
In January, the February expiration options are the front month. They will have higher theta decay than March or April expirations.