What do 'a' and 'c' stand for in 'Haversine formula' to measure the distance between two points?

Question

var R = 6371e3; // metres 
var φ1 = lat1.toRadians(); 
var φ2 = lat2.toRadians(); 
var Δφ = (lat2-lat1).toRadians();
 var Δλ = (lon2-lon1).toRadians(); 

var a = Math.sin(Δφ/2) * Math.sin(Δφ/2) + Math.cos(φ1) * Math.cos(φ2) * Math.sin(Δλ/2) * Math.sin(Δλ/2); 

var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a)); 

var d = R * c;

Here in this code formula is used in JavaScript. But anyway, there is a and c used in the original formula as well. What do they mean?

Answer 1

In fact, c is the angular distance between two points (in radians), and a is the square of half the chord length between two points.

P. S. See here.