What Ahosi kamma really is? The definition of ahosi kamma is defunct kamma which no longer bears fruits. As we know every action has its consequence. From Buddhist point of view, AFAIK, it's impossible to remove any vipaka completely. If an "old-kamma" can become exhausted, then what is the difference with no consequence? 

If a kamma is inhibited by a more powerful one, then the consequence is still there, maybe it is minimized or delayed. But if an action has no consequence, I think it's hard to understand it from the law of cause and effect point of view.
What Ahosi kamma really is?


Imagine you have a seed. That seed has the possibility to turn into a tree but only if the right conditions are met. 

For example, the seed needs water, fertile soil and maybe a little fence around it so that people will not step on it. If the seeds grows into a small young tree, then again it needs certain conditions to continue growing. For example it needs fertile soil, sunlight, water and again a little fence around it, so that cattle won't eat its leaves.

Its exactly the same case with Kamma. Kamma needs the right conditions to ripen and thereby come to fruition. If these conditions are not met, then the Kamma will not ripen and it will instead become Defunct (ahosi) Kamma.

For example, in the case of an Arahant. All their accumulated kamma from the past which was due to ripen in future lives, will become defunct when the Arahant passes away, since he/she will not get reborn. The Arahant has destroyed all fetters and cut all roots and thereby all causes and conditions for the past kamma to ever come to fruition.