A cenotaph is an "empty tomb" or a monument

Monument

A monument is a type of structure either explicitly created to commemorate a person or important event or which has become important to a social group as a part of their remembrance of historic times or cultural heritage, or simply as an example of historic architecture...

 erected in honour of a person or group of people whose remains are elsewhere. It can also be the initial tomb

Tomb

A tomb is a repository for the remains of the dead. It is generally any structurally enclosed interment space or burial chamber, of varying sizes...

 for a person who has since been interred elsewhere. The word derives from the Greek κενοτάφιον = kenotaphion (kenos, one meaning being "empty", and taphos, "tomb"). Although the vast majority of cenotaphs are erected in honour of individuals, many of the best-known cenotaphs are instead dedicated to the memories of groups of individuals, such as the lost soldiers of one country or empire.
Cenotaphs have been erected in many countries around the world since ancient times.