Pay-in day is the day when the securities sold are delivered to the exchange by the sellers and funds for the securities purchased are made available to the exchange by the buyers.
Source: Chapter 8, Section 8.3 – Clearing & Settlement
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The examiner expects the exact definition from the textbook. Note the distinction: Pay-in = sellers deliver securities + buyers provide funds to the exchange. Don't confuse it with Pay-out day (when exchange delivers to buyers/sellers). For 1 mark, one clear sentence is sufficient.