HBS Working Knowledge has an article by David Harding and Sam Rovit:
Every deal your company proposes to dobig or small, strategic or tacticalshould start with a clear statement how that particular deal would create value for your company. We call this the investment thesis. The investment thesis is no more or less than a definitive statement, based on a clear understanding of how money is made in your business, that outlines how adding this particular business to your portfolio will make your company more valuable. Many of the best acquirers write out their investment theses in black and white. Joe Trustey, managing partner of private equity and venture capital firm Summit Partners, describes the tool in one short sentence: “It tells me why I would want to own this business.”
A credible investment thesis should describe a concrete benefit, rather than a vaguely stated strategic value.