If 1 is primary, 2 is secondary, 3 is tertiary, what is 0?

Historically no such word exists. Primary, secondary, etc. come from Latin and ancient Rome. The Romans however, had no concept of 0. The closest they knew was 'none' (English translation). There is an old joke about this very question, to which the answer is "nunnery"/"noneary" :)

In recent years the term 'nullary' has won popularity in certain technical areas, mainly mathematics and computer science. Although the word exists, generally it doesn't make sense outside the particular field, because "There can be none before the first" (meaning: primary/secondary/etc. are labels you apply in order, if nullary were a word, it would label a thing that isn't there. But a thing that isn't there cannot be labeled.)

If you think of Primary, Secondary, etc.,as events, then zero precedes all events, but is not itself an event. It is a lack of events. It defines the state of the system prior to any events, so it is not comparable.

"O" would be the entirety of the entity composed of primary, secondary, tertiary and so on multiple aspects.