Every failed ontology project I have seen was described afterwards as a technical failure. None of them were.

The disagreements are real

Sales counts a customer at the signed contract. Finance counts at the first invoice. Support counts anyone with a login. All three are correct for their purpose, and forcing a single definition means one department loses a number they report on. That is not a modelling problem; it is a governance decision that modelling makes visible.

How to proceed anyway

• Model the distinct concepts separately and relate them, rather than forcing one term to serve three meanings.
• Name the owner of each definition. An unowned term drifts back into ambiguity within a year.
• Start with the handful of entities that appear in more than one system. Nobody needs a complete ontology, and attempting one is how these projects die.