null is a reserved constant used in Java and similar programming languages that is used to represent a void reference. In simpler terms, it is a value used to represent that a reference variable has no set value. In RuneScape it usually appears in string values either due to a variable not being assigned in time, usually due to lag, or as a result of a glitch.
It first made its appearance in game as a player name some people would see from time to time in RuneScape Classic. This can occasionally appear when moving between areas like going up and down stairs. This usually occurred when players with dial-up, or similar low speed, internet logged in and tried to type something.
An item also can be called "null". It means the same at the top of the page, the information of the object didn't load. Strangely the "null" object's name never changed. Previously, if one typed in 'null' on the quick chat system, this item could be said. This was removed in a hidden update.
If you search bank item "null" you get a message stating a number of search results equal to the number of empty spaces in your bank. This is explained by banks being represented as a List of items of some sort where empty slots are a null reference and occupied slots are references to the items they hold.
Sometimes, when making items, "null" is displayed for a split second in the "How many would you like to make?" interface. This tends to happen more often while smelting. Also, null can be made by dropping an object, then right clicking the object again before it is removed from your inventory, click "Use" and right click on another object.
Sometimes, immediately after logging in, if a player talks, or the game makes the player say something, the player's name will appear as 'null' in the chat box.
In Legacy of Seergaze, players repair a furnace. For a time after the quest was released, players could stand on the story above, and see the furnace through a hole in the floor. They could right-click on the top of the furnace (which registered as non-interactive scenery), and the menu would include the option "Examine null". This was later fixed by Jagex.
Adventurer's log Null
When Livid Farm was released, the Adventurer's log became glitched resulting in all of the entries reading "null". Examples include:"Item Found : Null.", "I killed null null", "I killed null null null null null null in Daemonheim." or other similar sentences. This happened for everyone with sentences containing a boss monster name, item names, or a completed treasure trail. For quest completions, it displays the title correctly, but then just says 'null'. This was finally fixed on 12 April 2011.