Hohmann Transfer
Definition
The most fuel-efficient way to move between two circular orbits — an elliptical path that just touches both, with a push at each end.
Technical definition
The transfer ellipse has periapsis at the inner orbit radius and apoapsis at the outer; two impulsive burns (Δv₁ to enter, Δv₂ to circularize) minimize total Δv.
Example
Going from low Earth orbit to geostationary orbit by Hohmann transfer takes about 5.3 hours of coasting and roughly 4.3 km/s of total burn.
Common confusion
A Hohmann transfer is fuel-optimal, not time-optimal. Faster transfers exist but burn far more propellant — missions choose based on whether time or fuel matters more.