Wyverns, sometimes spelled wivern, are a specific type of draconic monster, usually depicted as creatures similar to typical western dragons without forelegs, leaving their wings and legs as their only limbs. Depending on the work, they may either be a specific type of dragon or creatures related to but distinct from true dragons.
Wyverns originate from the heraldry of the British Isles, where since the sixteenth century a distinction was drawn between dragons with two legs and dragons with four (unlike in continental Europe, where this distinction was not typically made). British folklore, however, tended to use "dragon", "wyrm", and "wyvern" fairly interchangeably, sometimes even for creatures with no wings or, indeed, no limbs of any kind at all.

In modern fiction, wyverns are usually distinguished from other types of dragon by a number of traits in addition to their limb count:

• They are usually bipeds in the same manner as birds and other bipedal dinosaurs, although they may also be shown walking on all fours like bats or pterosaurs.
• They are typically venomous in some form; they're most often given a venomous, scorpion-like stinger at the end of their tails, but a venomous bite or toxic breath may also appear instead of or alongside stinging tails. Note here that "wyvern" comes from the same linguistic root as "viper".
• They tend to be less intelligent and more animalistic than other types of dragon, rarely being capable of speech even when "true dragons" speak fluently. Even when capable of speech, they tend to be portrayed as fairly stupid.
• They rarely if ever possess a breath attack outside of the aforementioned toxic breath, and are rarely depicted with hoarding treasure.
• They're generally a good deal weaker than true dragons (if they have any advantage at all, it will usually be in agility). Game settings home to both typically have wyverns as common Mooks, mounts, or wandering monsters, while Western and Eastern dragons tend to show up as powerful, unique bosses and campaign-ending villains.

A major reason for these differing associations likely comes down to the fact that, in real life, the classical six-limbed dragon body plan does not exist among any vertebrate species. That's not to say the wyvern body plan isn't implausible in its own way (that tail is basically nothing but drag), but it's much easier to sell as a creature that could actually exist. Because of this, the four-limbed dragon is by nature making concessions to biological reality, and therefore is likely to be less a creature of magic and more an unusually strong and dangerous animal.
Note that it is not uncommon in modern fiction for dragons to have this body plan but still be described as dragons and not really fit most of the other criteria, such as low intelligence, birdlike posture, and so on.

Compare other recurring types of dragons or draconic creatures, such as drakes (sometimes used for quadrupeds with weak or non-existent wings) and amphipteres (serpentine, winged and legless), which may sometimes be classified alongside wyverns as part of a category of "lesser dragons".

Etymology

The word "wyvern" is a development of Middle English wyver (attested in the fourteenth century), from Anglo-French wivre, which originates from Latin vipera, meaning "viper", "adder", or "asp". This aligns with earlier Germanic tradition, where dragons are portrayed as large, venomous serpents and are often called "wyrms". The wyvern is thus directly related to the French vouivre and guivre, and, by extension, also the Central European lindwurm.
The concluding "–n" had been added by the beginning of the 17th century, when John Guillim of Minsterworth, Gloucestershire, in 1610 describes the "wiverne" as a creature that "partake[s] of a Fowle in the Wings and Legs ... and doth resemble a Serpent in the Taile". John Gibbon, in 1682, emphasises that it "hath but two Legs".

Examaple of a Wyvern: Flame from LEGO Ninjago.