What is the (“spaceship”, three-way comparison) operator in C++?

This is called the three-way comparison operator.

According to the P0515 paper proposal:

There’s a new three-way comparison operator, <=>. The expression a <=> b returns an object that compares <0 if a < b, compares >0 if a > b, and compares ==0 if a and b are equal/equivalent.

To write all comparisons for your type, just write operator<=> that
returns the appropriate category type:

• Return an _ordering if your type naturally supports <, and we’ll efficiently generate <, >, <=, >=, ==, and !=;
  otherwise return an _equality, and we’ll efficiently generate
  == and !=.

• Return strong if for your type a == b implies f(a) == f(b) (substitutability, where f reads only comparison-salient state
  accessible using the nonprivate const interface), otherwise return
  weak.

The cppreference says:

The three-way comparison operator expressions have the form

lhs <=> rhs   (1)  

The expression returns an object that

• compares <0 if lhs < rhs
• compares >0 if lhs > rhs
• and compares ==0 if lhs and rhs are equal/equivalent.