Completion rate
Also called drop-off rate, partial response
The share of people who started your survey and reached the end; its inverse is drop-off.
Completion rate uses survey starts as its denominator. Track it during collection and inspect drop-off by page or question to find parts of the survey that may need attention.
Partial responses are what make that visible. If answers are saved at each page boundary, somebody who abandons halfway has still told you about the first half and shown you exactly where they stopped. Discarding partials throws away both.
Length is the main lever, and question type matters more than count: one open-ended question can cost more finishers than five choice questions.
Where it goes wrong
Reporting completion rate as if it were response rate. Ninety percent of starters finishing sounds excellent and says nothing about the people who never started.