The word softail is a registered trademark of Harley-Davidson, coined for the FXST Softail in 1984. Softail has become a genericized trademark, referring to other brands of motorcycles with hidden rear suspensions, and extended to bicycles as well.
The Harley-Davidson softail frame is designed to look like the rigid frame bikes of the past, while still having the comfort of rear suspension. The shock absorbers are positioned along the axis of the motorcycle, tucked under the transmission on models from 1984 to 2017 and under the seat on 2018 models.
There have been many Harley-Davidson models with the Softail frame, including the Softail Standard, Custom, Springer Softail, Heritage Softail, Heritage Springer, Night Train, Deluxe, Deuce, Fat Boy, Softail Slim, the Dark Custom Cross Bones, the Dark Custom Blackline and Breakout. With the exception of the Deuce, which has a 2 in (51 mm) longer backbone, these motorcycles have the same engines, transmissions, and frames, differing mainly in the fork, wheels, and accessories.
The Softail model line has multiple front fork configurations. Historically, they have had Springer leading link forks, reminiscent of the sprung front-ends that were used before the Hydra-Glide in 1949. The FXST designation is used for 21" front wheel bikes or when the Springer fork is used with a 21" wheel, while the FLST designation is used for 16" front wheel bikes or when the Springer fork is used with a 16" wheel.
Because Softail models do not have rubber-mounted engines, they have more vibration in Evolution-engined Softails than in rubber-mounted Touring or Dyna models with Evolution engines. To compensate, later models used a counterbalanced version of the Twin Cam engine instead of the regular Twin Cam engine used in the Touring and Dyna models. The Twin Cam was later replaced by a variant of the Harley-Davidson Milwaukee-Eight engine for the 2018 model year.
Bill Davis, an avid Harley rider and engineer from St. Louis, Missouri, designed the softail in the mid-1970s. His first design, which he worked on in 1974 and 1975, had a cantilever swingarm pivoted at the bottom and sprung at the top with the springs and shock absorber hidden under the seat. Davis then built a prototype based on his 1972 Super Glide. He patented the design and arranged to meet Willie G. Davidson August 1976. Davidson was impressed, but made no commitments, and six months later, said Harley-Davidson was interested but would not use the design at that time.
Davis continued to develop the design, switching the pivot and the springing points around so that the springs and shock absorber were under the frame and the pivot point was at the top of the triangular swingarm. This allowed the traditional Harley-Davidson oil tank to be placed under the seat. Davis attempted to produce the new design independently as the Road Worx Sub-Shock, but the partnership he had put together for this purpose collapsed. Harley-Davidson executive Jeffrey Bleustein contacted Davis shortly afterward and began negotiations to buy Davis's design. Davis sold his patents, prototype, and tooling to Harley-Davidson in January 1982. After further testing and development, Davis's design was introduced in June 1983 as the 1984 Harley-Davidson FXST Softail.
In April 1980, Harley-Davidson started work on its own rear suspension design that would have the look of a hard-tail motorcycle. The job was given a low priority until later in the year it was transferred to Jim Haubert Engineering, a firm that Harley-Davidson contracted yearly to custom build motorcycles and prototypes. Haubert built a prototype using his own rear suspension design that closely followed the look of the earlier Harley rigid frames. This version was completed enough for review by Harley-Davidson in January 1981.[better source needed]
In 2017, Harley unveiled a completely redesigned Softail frame for the 2018 model year, the first major change since the introduction of the Twin Cam engine in 2000. The 2018 Softail frame uses a differently shaped swingarm suspended by a single rear shock absorber, mounted underneath the seat in a similar fashion to the original Haubert and Davis designs. Harley claims that the new chassis is significantly stiffer and lighter than the previous-generation Softail and Dyna platforms, the latter being discontinued with some of its models being carried over to the new Softail chassis. The 2017 Street Bob 103 ci was dyno tested and made 65 hp (48 kW) and 88 lb⋅ft (119 N⋅m) at the rear wheel. The new 2018 Street Bob 107 ci makes 77 hp (57 kW) and 101 lb⋅ft (137 N⋅m) at the rear wheel. The 114 ci motor such as in the 2018 Heritage Classic produces 81 hp (60 kW) and 108 lb⋅ft (146 N⋅m) at the rear wheel. 
The Softail is the most popular Harley-Davidson model family for heavy customizing and is often used for show bikes. Many aftermarket manufacturers around the globe have specialized in Softail parts & accessories like wheels, fenders or tanks to change the look of the custom motorcycle. Harley-Davidson also started an Inspiration Gallery to show their latest factory custom parts on all new models  - many have their origin in CVO Softails, which are handmade Limited-Editions of the regular models.
Other softail-style motorcycles: