A Holter monitor is a portable device used to continuously record the electrical activity of the heart over a period of 24 to 48 hours. It is typically worn by patients to help diagnose irregular heartbeats or other cardiac conditions. The monitor consists of electrodes attached to the skin and a small recording device that captures heart rhythms during daily activities. Patients are usually instructed to keep a diary of their activities and any symptoms experienced while wearing the monitor. After the monitoring period, the data is analyzed by a healthcare professional to identify any abnormalities in the heart's rhythm, aiding in the diagnosis and treatment of conditions like arrhythmias or coronary artery disease.