@undesired

The word **rAjA राजा (from **rAjan- राजन् + /su) is necessarily a correct Sanskrit word regardless of anything grammarians might say, because it is correct according to current usage of all speakers, and because it has been used a gazillion times in a literary tradition that is at least 3000 years old.

Now suppose that we find some grammar rules that say, or seem to say, that rAjA राजा should be rAjI राजी. Whenever we find such a strange thing, the proverb "even if the holy scriptures say that fire is cold, fire still burns" comes to help.

If our rules say rAjI राजी and custom says rAjA राजा, we must automatically suspect that we are understanding the rules wrongly, and if that is not the case, that the rules are wrong, because the tradition of rAjA राजा is so weighty as to crush all grammar. That rAjI राजी is said to be an "undesired form", or "monster".

We have to find out some excuse to explain why the rules that make the monster are. If we can't find any, in extreme cases, we will say that /pANini was wrong and that we must ignore that rule. Most grammarians will rather kill their firstborn than say such a thing.