Ad space refers to the area on a website where an ad is displayed. Ad space is typically defined by the size of the ad that will be displayed in it, such as 300x250 pixels or 728x90 pixels. Ad space is often used interchangeably with the term "ad slot," but they are not the same thing.

To understand the difference between ad space and ad slot, imagine a website with several areas where ads can be displayed. The specific areas on the website where ads can be placed would be the ad slots, and the specific areas within those ad slots where the ads would be displayed would be the ad space.

Ad space is an important concept in the online advertising industry because it determines the size and placement of ads on a website. Advertisers and publishers must carefully consider the available ad space on a website when planning their ad campaigns, as this will affect the visibility and effectiveness of the ads. Ad slots are an essential part of the online advertising ecosystem, as they provide the space on a website where ads can be displayed.

One way to think about the relationship between ad space and ad slots is to compare them to the relationship between a picture and the frame in which it is displayed. The frame would be the ad slot, and the picture inside the frame would be the ad space. Just as a frame provides a structure and a border for a picture, an ad slot provides a space on a website where an ad can be displayed.