What’s in a name?

The development and evolution of the “Einstein–Æther Theory” (Æ-theory) shows that there is a field in cosmology where the word ether is being used again. It is unclear, however, whether this æther may be regarded in continuation with previous ethers, or it is an altogether new entity. The main goal of this paper is to understand the nature of this new ether in the context of previous instances of this scientific object. In order to do so, we shall first give a brief historical account of the distinct uses the word had assumed in the late nineteenth and early twentieth centuries, before its demise. Then, we shall describe the major attempts to revive the ether over the last century, focusing on the last endeavor: the Æ-theory. In this article, we do not intend to support or reject this new use of the word, but to stress the complexity of establishing a consistent historical narrative of some scientific objects like the ether.