These are the definations:

Test plan: A document describing the scope, approach, 
resources and schedule of intended test activities. It 
identifies amongst others test items, the features to be 
tested, the testing tasks, who will do each task, degree of 
tester independence, the test environment, the test design 
techniques and entry and exit criteria to be used, and the 
rationale for their choice,and any risks requiring 
contingency planning. It is a record of the test planning 
process.
Test strategy: A high-level document defining the test 
levels to be performed and within those levels for an 
organisation or programme (one or more projects).
Test Scenario:A document specifying a sequence of actions 
for the execution of a test. Also known as test script or 
manual test script.
Test Case: A set of input values, execution preconditions, 
expected results and execution
postconditions, developed for a particular objective or 
test condition, such as to exercise a
particular program path or to verify compliance with a 
specific requirement.
Test Script: Commonly used to refer to a test procedure 
specification, especially an automated
one.