What is the difference between a waypoint and a significant point in an airspace?
According to ICAO PANS-ATM (Doc 4444) 16th Edition,
Waypoint is a type of significant point.
Mentioned in Chapter 1 - Definition as

Significant point :- A specified geographical location used in defining an ATS route or the flight path of an aircraft and for other
navigation and ATS purposes.  Note.— There are three categories of
significant points: ground-based navigation aid, intersection and
waypoint. In the context of this definition, intersection is a
significant point expressed as radials, bearings and/or distances from
ground-based navigation aids.
Waypoint is defined as a specified geographical location used to define an area navigation route or the flight path of an aircraft
employing area navigation.

So in short waypoint is a significant point used to define an area navigation route or the flight path of an aircraft employing area navigation