A chest strap is a strap worn around the chest to secure something in place. It is often used to hold equipment or to measure heart rate during exercise. Full definition
However, you can still find several heart rate monitors with chest straps. (heavy.com)
You can also integrate the display with wireless heart rate chest straps, which helps you to target certain heart rate ranges during your workout. (geekwrapped.com)
No matter which fitness tracker you're using, you should know that wrist - based heart rate sensors aren't going to be as accurate as chest straps. (androidauthority.com)