The component of the electrocardiogram (ECG) that corresponds to the depolarization of atrial myocytes. Atrial depolarization precedes atrial contraction.

QRS complex
T wave

A rectangular signal waveform used in physiological studies to perturb (i.e., challenge) the system under study. The response of the system to the pulse is then studied carefully to learn about how the system responds to challenges.

Examples include pulses of voltage or current in electrophysiological experiments. Other examples include pulses of light, pressure, temperature, ligand, etc.

A square-wave pulse is defined by the amplitude and duration of the pulse, as well as by the frequency at which the pulse is applied to the system under study.

Neuronal Action Potential - Introduction

Square wave (Wikipedia)

The component of the electrocardiogram (ECG) that corresponds to the repolarization of the ventricles. Therefore, the T wave also corresponds to relaxation of the ventricles.

P wave
QRS complex