Buddhism says that all existence is ‘conditioned’ — it is all conditional on other conditions. Lord Buddha said:

All conditioned things are impermanent.

Everything is interconnected. Everything affects everything else. Everything that is, is because other things are. The fact that everything is nothing more than a set of relations is consistent with the modern scientific view of the material world. What is happening now is part of what happened before, and is part of what will happen next. It’s hard not to notice similarities with Einstein’s theory of relativity. Isn’t it? Lord Buddha explained:

When there is this, that comes to be;
with the arising of this, that arises.
When there is not this, that does not come to be;
with the cessation of this, that ceases.

Cambridge online dictionary defines impermanence as “not lasting forever or not lasting for a long time”. In Wikisource, the word is defined as “The quality or state of being impermanent.” Both of these definitions make sense. It is a very important concept of Hinduism and Vedanta and is one of the essential doctrines or three marks of existence in Buddhism.

In Sanskrit, it is called Anitya (अनित्य). The term Anitya appears in the Katha Upanishad, one of the Principal Upanishads. In Bhagavad Gita (Chapter 9, Verse 33), we see Lord Sri Krishna is saying to Arjuna:

kiḿ punar brāhmaṇāḥ puṇyā
bhaktā rājarṣayas tathā
anityam asukhaḿ lokam
imaḿ prāpya bhajasva mām

Translation: How much more this is so of the righteous Brahmanas, the devotees and the saintly kings; therefore, having come to this impermanent, miserable world engage in loving service unto Me.

The Impermanence doctrine of Buddhism asserts that all of conditioned existence, without exception, is “transient, evanescent, inconstant”.  All that arises, ceases.

In a letter written to the Maharaja of Mysore on June 23, 1894, Swami Vivekananda said:

This life is short, the vanities of the world are transient, but they alone live who live for others, the rest are more dead than alive.