What is SHVA in the Gmail's address bar: mail.google.com/mail/?shva=1#inbox ?

The answer is the following:

SHVA is an acronym for "Should Have Valid Authentication" (Gmail security).

The most notorious is the word Should.

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Other abbreviations:

SHVA Satellite Home Viewer Act of 1988
SHVA Sustained High-Voltage-Activated Current
SHVA Security Host Verification and Authentication