ISTQB Glossary definition
"A high-level document defining the test levels to be performed and the testing within those levels for a programme (one or more projects)."
In Simple English,
It is a published agreement to keep all parties involved informed about the strategy defined to test a project. This involves documenting the level of testing targeted, the kinds of testing agreed on as needed and the approach agreed upon.Field Notes
1. This document is often confused with test plan document.
2. Depending on the way on organization defines it, test strategy sometimes will involve several items that are generally considered as part of a formal test plan.
3. Some companies altogether combine both the test strategy and test plan documents and have a single artifact. They call it Master test document or test strategy or test plan.
remember, as rose by any other name is still a rose.
4.This however is different from test policy. While Test Policy is defined and maintained at the organization level, test strategy is at a project or program level. (Program here does not mean a piece of code. It means a very big Project which in turn has several projects under it)