What is the definition of situational awareness? In which phase of flight is situational awareness the lowest, and why?
A simple definition of situational awareness that you are aware of the different situation happening around you, know why they are happening and can think of an action you need to take to tackle with those.
In aviation, situational awareness means:
For a pilot, situational awareness means having a mental picture of the existing inter-relationship of location, flight conditions, configuration and energy state of your aircraft as well as any other factors that could be about to affect its safety such as proximate terrain, obstructions, airspace reservations and weather systems. The potential consequences of inadequate situational awareness include CFIT, loss of control, airspace infringement, loss of separation, or an encounter with wake vortex turbulence, severe air turbulence, heavy icing or unexpectedly strong head winds.
As far as your second question, hypothetically speaking, one can say that going through the preflight checklist, pilots are more focused inside the cockpit mostly and the situation outside the airplane is not too much of concern until they pay attention to that.
Generally speaking, when the airplane is not moving, situational awareness can be lesser. It is same as when you are driving, at what time you have lesser situational awareness?