Titthiya is almost universally translated as “one who belongs to another sect”, or some similar expression involving the term “sect”. The problem with this is that the titthiyas describe the adherents of any spiritual path apart from the Buddhist, including, for instance, the Jains. At present we would call Jainism a religion, not a sect. It seems to me, therefore, that titthiya should rendered as “one who belong to another religion”.
In fact, the word sect normally implies an subset of a larger religion, often a non-conformist group establishing their own orthodoxy. Now there is a Pali word, quite frequently used in the Vinaya Piṭaka, that almost perfectly matched this description, namely nānāsaṃvāsaka. Nānāsaṃvāsakas are Buddhist monastics who get ejected either for holding a wrong view or for refusing to abide by the monastic rules and regulations. They are, in effect, sectarians. Alternatively, nānāsaṃvāsakas are Buddhist monastics who disagree with the opinions in a certain monastic community and then formally separate themselves from those monastics. This, too, is a kind of sectarianism.
So, by freeing up the word sect by not using it for titthiya, it can now be used for nānāsaṃvāsaka instead. I now render nānāsaṃvāsaka as “one who belongs to a different Buddhist sect”.