Palindrome
A palindrome is a word, phrase, number or other sequence of characters that reads the same forwards as it does backwards.
LEVEL, ROTATOR and NOON are palindromes letter for letter. Longer ones are allowed to ignore spaces, punctuation and capitals, which is how "A man, a plan, a canal: Panama" and "Never odd or even" qualify. The convention is not laziness — it is what makes palindromes in ordinary sentences possible at all.
Among single English words, detartrated is often cited as the longest, and tattarrattat, coined by James Joyce in Ulysses for a knock at the door, is the longest in a major dictionary. Dates and numbers can be palindromic too, which is why certain calendar days get a small burst of attention.
A near relative is the semordnilap, itself palindrome spelled backwards: a word that becomes a different word when reversed, like STRESSED and DESSERTS. In crosswords, a palindromic answer is a gift to a setter, because a reversal clue can point at it from either direction and still be honest.