electrical conduction through a gas in an applied electric field
"The high-voltage wire sparked across the gap, creating a bright blue arc that illuminated the foggy night."
a continuous portion of a circle
"The rainbow formed a perfect arc across the sky after the rain stopped."
That part of a circle which a heavenly body appears to pass through as it moves above and below the horizon.
"The editor sent me an arc of the new thriller so I could finish my review by Friday."
Acronym of advance reader's copy, a copy of a book given to a reviewer free in advance for review purposes
In plain English: An arc is a curved line that connects two points without going straight between them.
"The rainbow formed a beautiful arc across the sky after the rain."
Usage: Do not use "arc" to mean an advance reader's copy; that is a specific publishing industry acronym unrelated to the word's standard meaning. Instead, reserve "arc" as a noun for describing a curved line or shape, or metaphorically for a character's journey of development in a story.
To move following a curved path.
"The gymnast launched herself into the air, arcing high above the mat before landing gracefully on her feet."
In plain English: To arc means to move along a curved path, like when something flies through the air in a bend before hitting the ground.
"The light would arc across the gap between the two poles during a lightning storm."
Usage: Use "arc" when describing an object that moves through the air along a smooth curve, such as a ball thrown upward or a rocket launching into space. Do not use it for straight-line motions or static positions.
Initialism of American Red Cross.
"The ARC distributed emergency supplies to families affected by the flood last night."
The word "arc" comes from the Middle English term for a bow or curve, which was borrowed from Old French and Latin. It ultimately traces its roots to an ancient Proto-Indo-European word meaning both "bow" and "arrow."