A large supermarket or similar retail outlet(noun)
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Use "megastore" in a sentence
"Established in 1971, the 13,500-square-meter megastore is an institution in Singapore, selling everything from saris to air tickets to canned chickpeas to Rolexes."
"It's the tinyiest Virgin 'megastore' I've ever seen."
""megastore," though a construction schedule has not been announced."