The term "viral video" is often misused. It should refer to a clip which has become popular through sharing on social media channels and word-of-mouth.
In reality the phrase is often used more in anticipation by marketers and PRs at launch ("hey, check out this great new viral video"), desperately hoping that coverage in the media will trigger or supplement the natural effect of web users simply enjoying a video clip.
Here are two examples to demonstrate:
On 29 April a Tupolev TU-154 aircraft was videoed in the sky above Moscow, seemingly out of control after being flown for the first time in ten years but encountering technical problems with its steering.
It's a terrifying clip. Luckily the pilot managed to land the aircraft at the Chkalovsky airfield on the second attempt. Inevitably the footage has spread around the web quickly and has since managed to attract almost 200,000 views just on this clip alone... in three days.
Meanwhile, British Airways is touting a new promotional video (uploaded to YouTube yesterday) to celebrate its Great Britons campaign, an interesting initiative to locate and nurture aspiring leaders in areas such as the arts, food and film, using the web as a starting point for people to register and share their ideas.
The campaign is fronted by top chef Heston Blumenthal, actor and writer Richard E Grant and Turner Prize nominated artist Tracey Emin.
One of these clips may go on to capture a million or so views across the web. The other might struggle a bit.