expression, noun-modifying (irregular)
so-called; what is called; what they call
Fixed prenominal expression used to introduce a term or name that is commonly known or referred to in a certain way. Often used in explanatory or descriptive contexts.
これが世に言う「一期一会」の精神だ。
This is the spirit of what is called 'ichigo ichie'.
彼は世に言う天才肌の研究者だ。
He is a so-called natural-born researcher.
いわゆる is the most common and neutral word for 'so-called'. 世に言う is more literary and emphatic, often used to introduce a well-known phrase or concept.
From 世 (yo, 'world, society') + に (ni, particle) + 言う (iu, 'to say'), literally 'what the world says'. The exact historical derivation is uncertain, but it has been used as a fixed expression since classical Japanese.