* Technically, the longest single-word palindrome in English is "racecar". It's a common word and is palindromic (reads the same backward and forward).
* However, you could consider proper nouns. In that case, "rotor" (a type of engine) is a contender, as well as "madam" and "level".
* You could consider technical terms. "Repaper" is a verb meaning "to cover with paper again" and is also palindromic.
Ultimately, the "longest" single-word palindrome depends on how you define "longest". "Racecar" is the most commonly recognized, but there are other possibilities depending on your criteria.