An object is said to culminate when it reaches its highest point in the sky.
Atmospheric conditions typically improve significantly with distance from the horizon, and so the time of an object's culmination is often the optimal time to observe it, though other considerations such as twilight may intervene.
When an object culminates, it lies on the line across the sky which connects the north and south cardinal points on the observer's horizon through the observer's zenith. This line is termed the observer's meridian.
For this reason, an object at culmination may equivalently be described as transiting the meridian, or often simply transiting.