What does "ultracold" mean?

Of a temperature close to absolute zero, especially one at which quantum-mechanical properties are observed.

What is a synonym for "ultracold"?

Common synonyms include: freezing.

adj, slang

adj, slang ·Uncommon ·College level

Adjective

1
Of a temperature close to absolute zero, especially one at which quantum-mechanical properties are observed.
2
Extremely cold; of utmost coldness. informal

"Physicists use ultracold atom facilities on Earth for a variety of experiments that probe the fundamental behaviors of atoms."

From ultra- + cold.