A Holter test, or Holter monitor, is a continuous portable electrocardiogram (ECG) that records the heart's electrical activity over 24 to 48 hours. Patients wear the device while engaging in daily activities to capture heart rhythms during normal conditions. It helps in detecting arrhythmias, intermittent ischemia, and other heart conditions that may not be evident during a standard ECG. The recorded data is analyzed to provide insights into heart health and is an essential tool in cardiac diagnostics.