nikgaukroger wrote:if you look into it more deeply it gets ever more messy. For example Livy uses Equites Catafractarii for what you'd call catafracts but later writers use clibanarii and it isn't clear when or why there was a change. There is also a rference to "catafractarii clibanarii" (or something close) in the later empire and, of course, dear old Vegetius uses "catafract" for infantry armour

From Henry George Liddell, Robert Scott, A Greek-English Lexicon on Perseus:
kata-phraktos , on,
A. covered, shut up, en desmôi S.Ant.958 (lyr., in old Att. form katapharktos ); ploia k. decked vessels, Th.1.10 codd., cf. Plb.1.20.13; en te tais aphraktois kai tais k. nausi IG12(1).41 (Rhodes, i B.C.); hê k. hippos cavalry clad in full armour, mailed, Plb.30.25.9, cf. Arr.Tact.4.1, 19.4; hippeis Plu. Crass.21 ; tak. coat of mail, PMagd.13.6 (iii B.C.): metaph., encased in ignorance of the future, psuchai Ion Trag.6.
So in general cataphract refers to being something being protected or guarded. A "cataphract" ship, e.g. trireme, is one which is decked and hence where the rowers are protected. The definition when applied to cavalry is essentially a derivative one, probably no more or less correct than when applied to infantry being armored or to ships.