On the Codex on Post Thumbnails, I see:
Post Thumbnails is a theme feature introduced with Version 2.9. It was quickly changed to Featured Images with Version 3.0. Post Thumbnail, now Featured Image, is an image that is chosen as the representative image for Posts, Pages or Custom Post Types.
On the changelog for Version 3.0:
Change Media UI labels from “Post Thumbnails” to “Featured Image”
Is there any difference in behaviour of post thumbnails and featured images, or was in 3.0 only the name changed? Why did the name change? It is a source of misunderstanding, as the image size is still called thumbnail or thumb, while it on the UI everywhere says Featured Image.
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This is explained in the core ticket #12554.
Change label in UI for Post Thumbnail and Page Image to “Featured Image” for both
We should change the UI label for the post thumbnail box on post and page editor screens. It’s post thumbnail for posts, and page image for pages, but it’s not always a thumbnail and the labeling is inconsistent. Let’s call it Featured Image for both, which is consistent and removes the size implication of the ‘thumbnail’ label.
