No strict definition, but I guess it means how fast a ship can go while still loading weapons, you don't need to load everything at once thought.
Regarding overloaded disrupters the only time you'd do that is if the ship is practically parked, at the higher speeds there isn't much shield reinforcement so normals are fine. We actually saw a lot of this in Klingon Civil War, many 3v3 battles would just be "close and hose" were we'd just fire at range 4. Quick, brutal, but the most effective you can get.
In Fed versus Klink it's very viable to close with overs if the fed is going to park.