Definition
A conjunction occurs when two bodies have the same ecliptic longitude (or right ascension). Inferior conjunction: inner planet between Earth and Sun. Superior conjunction: inner planet on the far side of the Sun.
Conjunction
Two bodies share the same ecliptic longitude as seen from Earth.
3D illustration
Advance time to see planets reach conjunction — same ecliptic longitude.
Use the menu and time controls to explore. Based on Jean Meeus' Astronomical Algorithms.