Semordnilap

Words that, when spelled backwards, produce another word.

Examples:

Room - Moor
Desserts - Stressed

Mood - Doom
Drawer - Reward
Time - Emit
Regal - Lager
Swam - Maws

Synonyms: "Semordnilaps are also known as heteropalindromes, semi-palindromes, half-palindromes, reversgrams, reversible anagrams, word reversals, or anadromes. They have also sometimes been called antigrams, though this term now usually refers to anagrams with opposing meanings." Source : Wikidictionary

Also sometimes known as an emordnilap. Some may consider semordnilap to be the plural form.

Q. If the queen buys a pint, does that make it a regal lager?
A. Dude, that's a semordnilap