Conditional evolution is a glitch effect which refers to the unexpected evolutions of Pokémon after battle, or outside of battle.
In Generation I
A non-freezing Super Glitch (containing a 0x50 sub-tile on the saved copy of the screen) will give the player the expanded party, and will likely cause unexpected evolutions if the Super Glitch effects are activated in a Pokémon battle and the battle finishes.
In Generation II
This causes a Pokémon called "ILSTY I PkPkPkHpPOKé ÄMn POKé" to evolve into a Forretress, followed by a Graveler named "?" evolving into Golem.
Ilsty and ?: