Hohmann Transfer Orbit

Because it moves a spacecraft between two orbits on the least propellant of any two-burn route, and carrying less propellant still matters more than arriving faster.

The Hohmann transfer orbit, described by the German engineer Walter Hohmann in 1925, uses exactly two engine burns: one to leave the starting circular orbit onto an elliptical path, and a second to circularize into the destination orbit. No other two-burn route between the same two orbits uses less propellant, and propellant mass is still the single biggest cost driver on an interplanetary mission, so the "textbook" route stays the practical one for missions to Mars even though faster, higher-energy trajectories exist. Its main real-world cost is timing: because it depends on the two planets lining up correctly, an Earth-to-Mars Hohmann transfer only has a launch window roughly every 26 months.