Anterior view of the heart*

[UPDATED] These two root terms mean "heart".

The first one, [-card(i)-] arises from the Greek [καρδιά] pronounced kardiá, and can be seen in medical words such as: cardiac, echocardiogram, cineangiocardiogram, cardioplegia, myocardial infarction, etc.

The second one [-cord(i)-] arises from the Latin [cor] or [cordis] and can be seen in words such as: precordial pain, cordial, commotio cordis, etc. 

As a point of interest, the original Greek spelling of [kardium]  was used by Nobel prize winner Dr. Willem Einthoven (1860 - 1927) when he invented the electrocardiograph and the electrocardiogram. The German term is [elektrokardiogramm] and the German abbreviation for the procedure is EKG. Since we use the term in English, [electrocardiogram] we use the abbreviation ECG. Both are correct, although if you are speaking English, ECG should be used.

* Image property of:CAA.Inc.. Artist:Victoria G. Ratcliffe