To solve the problem of identifying the most appropriate definition of the origin point in an AutoCAD 2D coordinate system, let's break down the options based on fundamental concepts of 2D coordinate systems, particularly as used in AutoCAD.

1. Definition of Origin Point:
   • In a 2D Cartesian coordinate system, the origin point is the point where the horizontal axis (X-axis) and the vertical axis (Y-axis) intersect.
   • It is denoted as \((0,0)\), meaning that both the x-coordinate and y-coordinate are zero.
   • This point serves as a reference point for locating all other points in the space.
   • In AutoCAD, the origin point \((0,0)\) serves as the default starting point for measuring all the coordinate locations and graphical elements within the drafting area.
2. Analyzing each option:
   • Option 1: Measured distance from the cursor to the previous location
     • This refers to dynamic measurement from one point to another but does not define an origin point.
   • Option 2: Measured distance from a fixed reference point
     • While a fixed reference point can be used in measurements, it isn't specifically defining the origin.
   • Option 3: 0,0 location
     • This is precisely the definition of the origin point in a Cartesian coordinate system used in AutoCAD.
   • Option 4: Effective diameter of the cursor on screen
     • This is unrelated to the concept of an origin point in a coordinate system.

Based on the reasoning above, the correct answer is indeed Option 3: 0,0 location as it accurately defines the origin point in AutoCAD's 2D coordinate system.