single file
adverb
- arrangement of items front to back; order that allows only one entity to pass through a point at a time
Wiktionary
adj
- One person wide.
“They walked in a single file line.”
“Single file traffic”
adv
- In a line or queue one person wide.
“They walked single file.”
noun
- A line or queue one person wide.
“They walked in a single file.”