It is an adjective that means "existing" or "in existence".
Here are some examples of how to use "existant":
* Il n'y a aucune preuve de la présence d'un dieu existant. (There is no evidence of the existence of a god.)
* Le système existant est inefficace. (The existing system is inefficient.)
* Le problème est que la solution actuelle n'est pas viable à long terme. (The problem is that the current solution is not viable in the long term.)