A natural arch, bridge or tunnel is a void beneath still standing bedrock, usually of a short extent, and allowing human passage from one end to the other, at least part of the time.
The other entrance/the other side is visible from each side.
A natural arch is an arch of rock formed by erosion (weathering) from the bedrock.
A natural bridge is a bridge of rock typically spanning a ravine or valley and formed by the erosion of a former cave.
A tunnel is a nearly horizontal cave open at both ends, fairly straight and uniform in cross-section.
As you can see, this definition is rather vague, which is actually a result of the bandwidth of different forms existing in nature. The definition tries to classify them in a way which is easy to understand and simple to determine. This is rather hard, because there are no numeric, measurable criteria. Even to measure the width, height, and length of an arch can get very complicated. See the link to The Natural Arch and Bridge Society below for more details and examples. And if you read their definitions and then the wikipedia page, you will see that there are numerous different definitions. On showcaves.com we decided to classify above three types as natural bridge. They all must go through a rock or cliff, which means two entrances and a (more or less) horizontal cavern/passage. In other words: you can not only walk through, you can see the sky on the opposite side.
There are numerous more complicated definitions of the different terms, and there are more specific names like Cliff Wall Arch versus Free-Standing Arch. Unfortunately, these definitions are often contradictory, as can easily be seen in the various proposals to distinguish between natural bridges and arches.
We are convinced that the last explanation is the most likely, one should never underestimate how big the influence of traditions is. It's similar with the term gorge versus canyon. We therefore do not distinguish between the two terms and use them synonymously or as proper names. Nevertheless, there are actually three geological processes creating bridges, a geological difference, based on their formation or genesis. Therefore, natural bridges can be divided into the following three types:
We were forced to paraphrase the three types, in fact there is no technical term for these three types of natural bridges, neither in German nor in English.