Is printer paper a4 or letter?

Is printer paper a4 or letter? What exactly is A4 paper, and why does it have a different name than the "standard" paper size?

Rite in the Rain All Weather Printer Paper, 8.5x11 Letter, No. 8511-50

Bond Paper Legal Size Printing Paper Writing Paper A4 Paper Office Paper A4 Letter Size Multipurpose Double a A4 Copy 80 GSM / White A4 Copy Paper - China Copy Paper, A4

What's The Difference Between US Letter and A4 Paper Sheets?

Letter (paper size) - Wikipedia

【No Paper Jams and Easy to Tear】 The Folded Thermal Paper has Jagged Edges Between Each Page, Making A4 Thermal Paper Easy to Tear Apart, and Foldable

PeriPage 100 Pages Folded A4 Thermal Printer Paper for Long-lasting, 8.5 x 11 Inch US Letter for A40 Portable Printer and Brother PocketJet

How to print an A4 printable onto US letter size paper (and vise versa)

Printer Paper Review

A4 vs Letter: What Is The Difference Between A4 And US Letter Size?

What's The Difference Between US Letter and A4 Paper Sheets?

What are the causes and effects of global divides? - Quora

COLORWING Thermal Printing Paper 8.5x11in - Multipurpose Paper Compatible for HPRT MT800 MT800Q Phomemo M08F and Brother PJ762 PJ763MFi Portable Printers, US Letter Size (200 sheets) : Office Products