But a backlash against the term and its implications is brewing. Unicorn is ‘‘the most overused word in the technology industry today,’’ the venture capitalist Mark Suster recently griped on his blog. Pessimists argue that the fact that so many companies, some of which have yet to see a dollar in profit, have such breathtaking valuations is a sign that the technology industry is living in a fantasy world. They say founders and investors are disconnected from reality, conjuring numbers out of thin air. When the current tech bubble inevitably bursts and the illusion is dispelled, the argument goes, the unicorn will be revealed as nothing more than a donkey with a cardboard tube taped to its forehead.

This sort of spirited debate has always surrounded the unicorn. The Greek physician Ctesias wrote of the unicorn in fourth century B.C., claiming that in India there lived ‘‘certain wild asses’’ with white bodies, dark red heads, dark blue eyes and, on their foreheads, a single horn. Later, scholars and explorers described unicorns in such wildly varying terms — fearsome and meek, hulking and small, black and white and multicolored — that experts have concluded the unicorn legend arose thanks to excitable Europeans mishmashing the characteristics of exotic animals like the oryx and the Indian rhinoceros in a sort of taxonomical game of telephone. The mystical power of the unicorn’s horn was the most common thread: It was believed to be capable of purifying tainted water, protecting against poison and treating the plague.

The lucrative trade in alicorns, which rested on this widespread belief in the unicorn’s power, reached its peak in the Renaissance amid a widespread fear among nobility of being poisoned by their rivals. They lined their cups with pieces of alicorn and placed whole horns on the table, believing that they would sweat in the presence of poison. (Think of the alicorn as Uber, for not dying of poison.) And as the centuries wore on, powdered unicorn horn made the antidote accessible to the masses: London pharmacists carried it in their inventories in the early 1700s. But by the late 18th century, the market became oversaturated, and the horn bubble popped just as the rise of skeptical inquiry prompted people to question the unicorn horn’s effectiveness and the existence of the unicorn itself. In retrospect, it was clear that centuries of wealthy nobles, people who wanted for almost nothing, had thrown away untold fortunes in their pursuit of these worthless baubles. No doubt, many modern-day unicorn hunters will wind up doing the same.

Silicon Valley’s appropriation of the unicorn is not the first in the new millennium. The sex columnist Dan Savage, responding to letters from heterosexual couples unable to find partners for ménages à trois, described their quarry as ‘‘unicorns’’: the rare bisexual woman who is game to join a straight couple in a no-strings-attached threesome. In his columns, tales of the unicorn often function as both a caution and an inspiration to the adventurous couples who seek one. Savage takes pains to remind them that, while traditional sex norms have weakened to the point that such an arrangement is increasingly possible in theory, an enlightened attitude by no means entitles you to the prize.