Question

What is meant by spacetime? What is an "event" in spacetime? What is a "world line"?

Answer 1

Space-time is a mathematical model that joins space and time into a single idea called a continuum. This four-dimensional continuum is known as Minkowski space. This consept is used in a relativistic universe, where time cannot be separated from the three dimensions of space as in relativistic model observed rate at which time passes depends on an object's velocity relative to the observer

Event :-
In physics, and in particular relativity, an event is a point in spacetime meaning a specific place and time of the physical situation or occurrence associated with it.
For example, an accident is an event which occurs at a unique place and a unique time.

Word line :-
The world line of an object is the path of that object in 4-dimensional spacetime, tracing the history of its location in space at each instant in time. It differs from from concepts of "orbit" or "trajectory" by time dimension !!
It can also be difined as a sequence of spacetime events !!