Anicca
All compounded phenomena (things and experiences) are inconstant, unsteady, and impermanent. Everything we can experience through our senses is made up of parts, and its existence is dependent on external conditions. Everything is in constant flux, and so conditions and the thing itself is constantly changing. Things are constantly coming into being, and ceasing to be. Nothing lasts.
The important point here is that phenomena arise and cease according to (complex) conditions. In Mahayana Buddhism, a caveat is added: one should indeed always meditate on the impermanence and transitory nature of compound structures and phenomena, but one must guard against extending this to the realm of Nirvana, where impermanence holds no sway. In this view, the ultimate nature of reality is free from the stains of dualistic thought, and should therefore not be labeled as 'one' or the 'other' (i.e. 'permanent' or 'impermanent').
Dzongsar Jamyang Khyentse Rinpoche states that in the four seals of the Mahayana, Nirvana should be viewed as "beyond extremes". Furthermore, he says "In many philosophies or religions, the final goal is something that you can hold on to and keep. The final goal is the only thing that truly exists. But nirvana is not fabricated, so it is not something to be held on to. It is referred to as 'beyond extremes.' We somehow think that we can go somewhere where we’ll have a better sofa seat, a better shower system, a better sewer system, a nirvana where you don’t even have to have a remote control, where everything is there the moment you think of it. But as I said earlier, it’s not that we are adding something new that was not there before. Nirvana is achieved when you remove everything that was artificial and obscuring."
Read more about this topic: Three Marks Of Existence