What is delta-v?
Also called: Δv
Delta-v is how much a rocket can change its speed — the real currency of spaceflight. Reaching low Earth orbit costs roughly 9,400 m/s once losses are included.
It is written Δv, "change in velocity". Every manoeuvre has a price in delta-v, and a vehicle has only as much as its propellant allows.
The Tsiolkovsky rocket equation gives it: Δv = Isp · g₀ · ln(m₀ / m_f). The logarithm is the cruel part — doubling the propellant does not double the speed.
That single term explains why rockets are enormous, why payloads are small, and why staging works so well.
See it happen
Push & GoWhy rockets flySource: Merlin rocket engine family