Webster's Revised Unabridged Dictionary · 1913

Visualize

Visualize , transitive verb

1.
To make visual, or visible.
2.
to see in the imagination; to form a mental image of.
No one who has not seen them [glaciers] can possibly visualize them. — Lubbock

Visualize , intransitive verb

To form a mental image of something not present before the eye at the time.