Just like the titles Paramahamsa and Siddha, Arihant also denotes spiritual progress. But why is someone called Arihant and at what stage of spritiual progress?
The earliest reference to Arihant is found in Rig Veda X.12.166 :
0 Rudra-like Divinity ! do thou produce amongst us, of high descent, a Great God, like Rishabha Deva, by becoming Arhan, which is the epithet of the first World Teacher; let Him become the destroyer of the enemies !
Arihant is very common in Jainism, in fact all their tirthankaras were arihant(Rishabha was 1st tirthankara). Similarly Buddhism defines Arihant as one who is a perfected being. But anyone who gets realization can be called a perfected being, in Advaitic terms, such a soul is called Jivanmukta and Paramahamsa.
So who exactly can be called Arihant? I think a Jivanmukta who decides to teach the world is called Arihant and once he leaves his body, he becomes Siddha.