Is Central America an isthmus and not the narrowest part of Panama?

According to the definition an isthmus is a narrow strip of land that connects two large masses of land. Both South and North America where two single large masses of land that later were connected by the small strip of land called now Central America which has been proven that in the most part is "new land" and not only a part of Panama. According to what I was taught long ago C.A. is an isthmus and not what many people believe now, a part of Panama.

I don't understand where the confusion lies. The Isthmus of Panama (the narrow strip of land that connects N. America and S. America) is, I repeat, is the Republic of Panama. The north end of the isthmus, which is the northern boundary of the Republic of Panama, connects to Costa Rica (the southernmost country in Central America). The south end of the isthmus, which is the southern boundary of the Republic of Panama, connects to Columbia (a country in South America). In conversation some people, erroneously, refer to the AREA AROUND the Panama Canal as the Isthmus. This is for brevity in conversation.

I think most of the region of Central America, excluding Panama, is 2 wide 2 b considered a isthmus.

There's nothing to get so technical about with it. I suppose you could be considered correct. First, Panama, part of Central America, certainly is an isthmus . C.A. as a whole could be seen as an isthmus, taken more broadly, due to the breadth of the Yucatan peninsula and South America, but usually what is refered to as "the" isthmus in that region is just Panama.

Central America, which comprises the countries of: Guatemala, Belize, El Salvador, Honduras, Nicaragua, Costa Rica and Panama, is a (geographical) region of the North American continent. The isthmus of Panama is the bottom half of the Republic of Panama. Hence why Panama is a trans-continental country. The "new land" that connected North America and South America is the bottom half of Nicaragua, all of Costa Rica and Panama.

Central America, in whole, is not an isthmus, when people say that they are referring to the southern part of the region, the bottom half of Panama.