Let me start by saying the image above is of a hot tub, not a Jacuzzi. Though, in theory, it people could refer to it as a Jacuzzi.
The reason is that Jacuzzi is a brand; a famous brand of hot tubs. It's kind of like how many people refer to a vacuum cleaner as a Hoover. The fact is that Hoover is a brand that makes vacumes but you may not own one of that brand.
So, Jacuzzi is just that, a brand of hot tubs.
Wikipedia has the following quoted on their page (I've also linked to this page above)
Founded in 1915 by seven brothers, led by Giocondo Jacuzzi, Jacuzzi and Brothers made wooden propellers under military contracts, based at 2043 San Pablo Avenue, Berkeley, California. In 1920 the brothers also dabbled briefly with aircraft design and manufacture, with a single-seat monoplane and a seven-seat cabin monoplane. Both aircraft were noted for their use of laminated wood products for fuselage manufacture, but were essentially unsuccessful, with only one of each type being built. In 1921, Giocondo Jacuzzi, a pilot, and two company employees were killed when one of their planes crashed in Modesto.By 1923 the company was styled as Jacuzzi Bros Propellers, with headquarters at 1450 San Pablo Ave.
The company survived to become famous as the whirlpool and bubble bath manufacturer of today, with the name Jacuzzi becoming synonymous with the bath products regardless of manufacturer.
So, hopefully, that makes it a little clearer. Throughout the posts on this website, I may refer through to inflatable hot tub, portable spa, Jacuzzi and other references as well. Please know that I am always using them to refer to hot tubs (potentially the brand) and there is no real difference in the functionality.