The verb form of vipassanā is vipassati, as found in such phrases as tato tattha vipassati (snp5.15:4.4), tattha tattha vipassati (mn131:3.6), bahiddhā ca vipassati (thag7.2:7.2), sammā dhammaṁ vipassato (thig3.8:2.4), anupadadhammavipassanaṁ vipassati (mn111:2.8), and so on.
When prefixes are added or subtracted from verbal forms in Pali, sometimes they make no difference and sometimes they make a great difference. We can only understand this by looking closely at the usage in context.
In most of these cases, vipassanā has a fairly vague sense, and as the tradition says, it is used in a way that is broadly similar to other terms for wisdom. See for example the lists of synonyms in the Abhidhamma (ds2.1.1:28.2 or ps1.1:520.2), or for example mnd16:49.2:
Bodhi vuccati catūsu maggesu ñāṇaṁ paññindriyaṁ paññābalaṁ dhammavicayasambojjhaṅgo vīmaṁsā vipassanā sammādiṭṭhi.
“Awakening” is said to be the knowledge of the four paths, the faculty of wisdom, the power of wisdom, the awakening factor of investigation of principles, inquiry, discernment, right view.
Vipassanā ca amoho ca aniccasaññā ca anattasaññā ca imāni cattāri padāni vipassanaṁ bhajanti
Discernment and non-delusion and perception of impermanence and perception of not-self: these four states are associated with discernment.
There is no attempt to make any special link between vipassati and passati.
Vipassati is part of a broad spectrum of terms sharing a semantic field. Some of those terms also share an etymological root (anupassanā, sampassati, samanupassati, and so on). But the question is, what is this distinctive term implying and what is its contextual purpose?
While the general meaning of vipassanā is similar to many other terms, when the EBTs use vipassanā in doctrinal contexts, they do so in a unique and highly distinctive way: the meditative development of wisdom (through contemplation of the three characteristics, etc.) as a complement to samatha, the meditative development of tranquility. No other term is used in this way.
Passati and its variants are, by contrast, used in a wide range of senses and do not have the specificity of vipassanā. Some of these uses overlap with those of vipassanā, while others do not. For example, passati is commonly used to denote realization (seeing the four noble truths at stream-entry), psychic powers, jhanas, ordinary seeing, and so on.
If we reduce the meaning of the specialized term vipassanā to the general term passati we are eliding meaningful distinctions and end up not being able to explain the thing we wanted to explain. There are reasons why the modern meditation movement is called vipassanā, not passanā or anything else. Those reasons evolved from how vipassanā is used in the early texts, as a specific term for the meditative development of wisdom in partnership with samatha.
The EBT teaching of the complementary qualities of samatha and vipassanā became, in the Commentaries, teachings on different meditation methods. In the 20th century those meditation methods became the foundation for different meditation schools.
The Burmese vipassanā schools, in particular, adopted a coherent and distinct set of doctrinal interpretations, defining themselves first and foremost as teachers of vipassanā, while de-emphasizing, sidelining, or outright rejecting the practice of samatha. These doctrinal interpretations are well known, and have always been contested in Theravada. It is to give a name to this set of of doctrines that I coined the term vipassanavāda.