When in French they are called carte postale the same was as in English. I also noticed that on some old European postcards they wrote "postcard" on several languages.
On the other hand to distinguish a postcard form just a a picture on a thick card-stock you may want to use the reverse side to put some information about the image or the company. Traditionally postcards also have some lines to make writing address easier.
Here are some examples of the reverse sides of the postcards.
The right side of the cards is for the address only.
On some cards they would explicitly state that.
This company's name is Valentine and sons, which is also remarkable for me because my father's name is Valentine.
Some reverse sides are very simple like this one.
Some are very pretty.
This reverse side of a soviet postcards form the 1957 has an interesting story to tell. In the Soviet Union price was printed on products by the manufacturers (the same way it is printed on books in the USA). You may see "20 коп" price tag on the right, which stands from 20 kopeek = 0.2 ruble. However in 1961 Soviet rubble was denominated 10 times, and you may see a later stamp from 1961 saying that the new price is "2 к".
On this american postcard they printed not the price of the postcards but the price of the postage.
The current price is 46 cents for United States and $1.10 for international, with no discounts for Cuba or Canada.
Returning back to our postcards, after looking on the reverse sides of old postcards we decided to keep it simple with the lines for address, logo, website address and title. Starting with the NC cards we also added the small silhouette of the state and year of printing.