A jammer is a style of swimsuit

Swimsuit

A swimsuit, bathing suit, or swimming costume is an item of clothing designed to be worn by men, women or children while they are engaging in a water-based activity or water sports, such as swimming, water polo, diving, surfing, water skiing, or during activities in the sun, such as sun bathing.A...

 used mainly in competition to obtain speed advantages. Many manufacture this suit including Speedo

Speedo

Speedo International Ltd. is a manufacturer and distributor of swimwear and swim-related accessories. Founded in Australia in 1914, the industry leading company is now a subsidiary of Pentland Group Plc. Today, the Speedo brand can be found on products ranging from swimsuits and goggles to wrist...

, Diana, TYR and Blueseventy. They are made of nylon and lycra/spandex material and have a form fitting design to reduce water resistance. They provide moderate coverage from the mid-waist to the area above the knee, somewhat resembling cycling shorts

Cycling shorts

Cycling shorts are short, skin-tight legwear designed to improve comfort and efficiency while cycling...

 or compression shorts

Compression shorts

Compression shorts are undergarments usually worn by athletes. They are form-fitting garments and when worn cover the athlete's waist to mid or lower thigh. These garments are often made from a spandex-type material, similar to cycling shorts....

 worn by many athletes. They provide greater leg coverage than swim briefs

Swim briefs

A swim brief, or racing brief, refers to any briefs style male swimsuit such as those worn in competitive swimming and diving. The popularity of the Australian Speedo brand racing brief has led to the use of its name to refer to any racing brief, regardless of the maker...

 and square leg suits

Square leg suits

The square cut or square leg style suit is a form-fitting male swimsuit used as a slightly more conservative style than swim briefs for water polo and diving, or for recreational wear. Like swim briefs, they are made of a nylon and spandex blend. They typically sit low on the waist and high on the...

, although they also have slightly more water resistance.

Certain makes can assist with buoyancy, helping the body maintain a more streamlined swimming position.