What does "neat" mean when ordering a drink at a bar?

What’s the difference between neat and on the rocks? How is straight up different from neat? Cocktail lingo has been around for decades but for a lot of people, the difference between these styles can be confusing or simply misunderstood. Neat is the least confusing of cocktail terms. This means that a spirit is directly poured into a glass (preferably a NEAT Glass). It’s similar to a shot, but the glass makes a huge difference in the sipping experience. Neat drinks are about two ounces, not chilled, there are no extra ingredients (even ice) and no, you can’t order an Irish Car Bomb neat. Brandy and whiskey are the most popular spirits to drink neat. High-quality spirits of any kind are commonly enjoyed neat as well.

Source: SIP AWARDS