⟨IDSP⟩
A blank exactly the size of an ideograph — the indent of the Japanese paragraph.
In CJK text every character fills a full square — and the space must fill the same. U+3000 is that blank square: Japanese paragraphs traditionally open with one.
CJK columns and forms align with it, not with Western blanks. No named entity — 　 — but any Japanese input method produces it with the space bar in full-width mode.
Narrow: ' 吾輩は猫である。' (Latin space: half a square)
Exact: ' 吾輩は猫である。' (U+3000: one full square)