Plain English Meaning
The lowest weather, mainly how far you can see down the runway, that allows you to legally take off when flying on an instrument flight plan. If the weather is worse than this, you cannot depart.
Full Definition
The minimum weather conditions, expressed as visibility and sometimes ceiling, that must exist at an airport before a pilot operating under instrument flight rules may legally begin a takeoff. Standard takeoff minimums for aircraft having two engines or less are 1 statute mile visibility, and for aircraft having more than two engines, 1/2 statute mile visibility. Specific airports may have higher, non-standard takeoff minimums published due to obstacles or in the .