A ladder in the 1930s
An access ladder on the USS Enterprise
A ladder in a library
A ladder was an object consisting of a series of rungs that allowed one to access different levels of various structures.
The launch bay aboard Enterprise NX-01 featured ladders to get into the shuttlepods. (ENT: "Broken Bow", "The Andorian Incident", "Impulse" et al.) Engineering was also equipped with several ladders. (ENT: "Breaking the Ice", "Fortunate Son")
The Jefferies tubes aboard NX-class starships were also equipped with ladders and a maintenance shaft can be reached from sickbay by a ladder. (ENT: "The Catwalk", "The Crossing", "Regeneration")
In 2151, Hoshi Sato surmised that the aliens aboard an unknown ship were bipeds, based on a ladder she noticed in one of the corridors. (ENT: "Fight or Flight")
Ladders were also installed in the cargo modules of Y-class freighters to reach the lower part of the cargo module from the catwalk. (ENT: "Fortunate Son")
Ladders can also be found in the corridors aboard a Vulcan D'kyr-type starship. (ENT: "Impulse")
Several ladders were present on various starships. They were also sometimes a component of fire escapes. (TOS: "The City on the Edge of Forever")
Rule 211 of the Rules of Acquisition stated, "Employees are the rungs on the ladder of success. Don't hesitate to step on them." (DS9: "Bar Association")