The heart has 4 chambers, two in the Atrium, two in the Ventricles. Heart cells respond to the minuscule electrical signal generated by certain specialized cells, but, unfortunately, those heart cells that typically respond to that electrical stimulus can also generate electrical signals themselves, causing the heart muscle to contract at an inappropriate time.

The normal electrical signals generated by the specialized cells to cause the heart to beat are located in the Atrium and is known as the SA (Sino-Atrial) Node. From there are fibers that travel to the other side of the Atrium to carry the electrical signal and there are fibers that travel down towards the Ventricles, actually going to the AV (Atrio-Ventricular) Node.

The AV Node is a merciful design, it's the reason that those of us with Afib aren't more impacted by it than we are. The AV Node acts to limit the rate that the Atrial Signals are conducted to the Ventricles. Afib can produce very high Atrial Rates, something like 300 Beats Per Minute, but the AV Node limits how many of those Atrial Signals get through.

But, normal signals start in the SA Node, travel to the other chamber of the Atrium. From the SA Node the signal also travel to the AV Node. From the AV Node the Signal Travels to the Ventricles, conducting cells distributing the signals to both chambers of the Ventricles.

SVT - Supra Ventricular Tachycardia (Supra means above) is a Tachycardia that results from a source above the Ventricles, as does the normal heart beat, but Tachycardia usually refers to a rate above 100 Beats Per Minute.

So, with that in mind are these definitions from an online source:

""• Sinus tachycardia
Normal sinus tachycardia. Sinus tachycardia is present when a person's heart rate is over 100 beats per minute. This is normal if there is a clear reason for the fast heart rate, such as exercise, pain, or fever. With sinus tachycardia, the electrical system of the heart is working normally.

• Abnormal (inappropriate) sinus tachycardia. Sinus tachycardia (heart rate over 100 beats per minute) is not normal if there is no apparent cause. In this rare condition, the electrical system of the heart is working normally.

• People with inappropriate sinus tachycardia typically don't have any heart disease. Inappropriate sinus tachycardia with no heart disease may mean your autonomic nervous system isn't working right.""

Sinus Tachycardia is a form of SVT, as it originates in the top of the heart, above the Ventricles.

Actually, after I wrote this, but still researching, I found this Mayo Page:
https://www.mayoclinic.org/diseases-conditions/supraventricular-tachycardia/symptoms-causes/syc-20355243
In part, it also covered this:

""Other types of supraventricular tachycardia include:

Sinus tachycardia""

If you have inappropriate tachycardia, times that your heart goes above 100 without reason, be sure to see your doctor. There are multiple reasons for Inappropriate Tachycardia, they should be evaluated and treated as needed.

Tachycardia can also be limited to the Ventricles, that's why I mentioned that cells can discharge on their own and cause other cells to beat. Sustained Ventricular Tachycardia (source of the fast rate is on the ventricles) can be life threatening and would require immediate emergency care. NSVT, Non-Sustaining Ventricular Tachycardia, my Cardiologist said even in healthy individuals, even college age ones, an occasional short episode has been recorded during studies. Nevertheless, it pays to consult your doctor for any irregular rhythm episodes until they can be recorded and diagnosed.