What is

[/] is the what is command. It has several functions for providing you with more information about the game. These functions include:
 * Activating the far look mechanic to examine things on the map
 * Displaying information about what a given ASCII symbol represents
 * Displaying the encyclopedia entry for a given symbol or topic

Usage
When you press [/], first you will be given an opportunity to far look using cursor select: '''Specify unknown object by cursor? [ynq] (q)'''
 * Choosing y will allow you to move to a symbol on the map using the movement keys
 * Choosing n will prompt you to input an ASCII character or topic
 * Choosing q will abort the command

Cursor select
If you choose to specify an object with the cursor, you will see: Please move the cursor to an unknown object. (For instructions type a ?) If you press [?] for movement instructions, the following help text is displayed: Use [2468] to move the cursor to an unknown object. Use [HJKL] to move the cursor 8 units at a time. Or enter a background symbol (ex. <). Type a. or, or ; or : when you are at the right place. Type Space or Escape when you're done.

ASCII symbol legend
Once you select a square, you will be given a brief description of the symbol there. For example: +    a spellbook or closed door (closed door) ... or ... d    a dog or other canine (tame dog called Fido) If your selection has an encylopedia entry, you will be able to get additional information. In this case you will see: '''More info? [yn] (n)''' You can also get the symbol legend for a character if you choose 'no' to cursor select, and then type a single character. In this case you will not be prompted for more information, because there could be different encyclopedia entries for the things that symbol represents.
 * Choosing y displays the encylopedia information for your selection
 * Choosing n lets you select another object with the cursor

Compare selecting a staircase with cursor select: <    a staircase up or a ladder up (staircase up) '''More info? [yn] (n)''' ... vs. typing '<' ... '''Specify what? (type the word) <''' <    a staircase up or a ladder up

Consulting the encyclopedia
Choosing 'yes' when asked for more info, or typing in a complete word or phrase when asked to specify what, will display the encyclopedia entry for that subject. Usually this is a lore reference from a real-world source (which may not be 100% accurate in NetHack), a brief writeup by the devteam, or even a funny quote. If there is nothing in the encylopedia for the phrase you entered, you will be told: I don't have any information on those things. The encyclopedia has information about a vast array of subjects. It is possible to be very specific when requesting information. Compare the following examples: '''Specify what? (type the word) fog''' I don't have any information on those things.

'''Specify what? (type the word) cloud''' I wandered lonely as a cloud That floats on high o'er vales and hills, When all at once I saw a crowd, A host, of golden daffodils; Beside the lake, beneath the trees, Fluttering and dancing in the breeze. [ I Wandered Lonely as a Cloud, by William Wordsworth ]

'''Specify what? (type the word) fog cloud''' The fog comes on little cat feet. It sits looking over harbor and city on silent haunches and then moves on. [ Fog, by Carl Sandburg ]

Strategy
This command can be quite helpful to new players still learning what some of the symbols represent. This is especially true when facing unfamiliar monsters. Cursor select can be used to tell if detected creatures are peaceful. It can also be helpful in verifying that you have acquired detection abilities such as telepathy -- this can be used to help identify an amulet of ESP or a ring of warning, for instance.