A reusable workflow can be packaged up as a workflow template and moved to a different site collection. The package is a workflow solution file with a .wsp extension. This file is deployed to the solution gallery and then activated so it can be used at the other site collection. The procedure is demonstrated in this video.
This video is part of the SharePoint Designer 2010 Workflows video tutorials DVD
Course: SP202 – SharePoint Designer 2010: Build Workflow Solutions
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