The best replacement for test plan is test case.

The best alternatives to test plan are rated as following:
★★★★★  test case
★★★★★  test strategy
★★★  test script
★★  test procedure
★★  test suite
★  test approach
★  test specification
★  test design
★  test cases
★  test scenario
  strategy
  test protocol
  validation plan
  test plan

Your alternative for test plan is not listed? Suggest!
Speaking about test plan, one probably shouldn't forget about entry and exit criteria.

Other search terms related to test plan and their ratings are:
★★★★★ entry and exit criteria
You have in mind a companion for test plan and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of test plan in all years. Click on graph for more.
test plan
test case
test strategy