ǵneh₃-mn̥-₂
“a mark of knowing, sign”gnomon, diagnosis, ignorant
Nominal of *ǵneh₃- giving Greek gnōmōn, English gnomon, diagnosis, prognosis, ignorant.
Discussion
The PIE form *ǵneh₃-mn̥- (a mark of knowing, a sign, a distinguishing feature) is a nominal derivative of *ǵneh₃- (to know, to recognise), formed with the *-mn̥- suffix producing a result noun: "that which results from knowing" — a mark, a token, a name by which something is recognised.
Latin nōmen (name) may be connected to this root through the concept of "that by which one is known," though the standard derivation of nōmen traces it to *h₁nómn̥ (a different PIE root specifically meaning "name"). The connection between knowing and naming is semantically transparent — to name is to know, to recognise is to call by name — but the phonological details are debated.
More securely, Latin cognōscere (to know, to recognise — from con- + gnōscere) and the derivative cognōmen ("the known-name" — a Roman's third name, the personal identifier by which they were individually recognised) connect to the base root *ǵneh₃-.
Greek gnṓrisma (γνώρισμα, "a mark by which something is known, a distinguishing feature") preserves a closely related formation — the sign that enables recognition.
The *-mn̥- suffix is the same instrumental/result suffix found throughout the PIE derivative vocabulary: *srew-mn̥- (stream, the thing that flows), *legʰ-mn̥- (that which is laid down), *sengʷʰ-mn̥- (that which is sung). In each case, the suffix converts the verbal action into its concrete result.