What feature allows part of an image to be made transparent so that objects placed behind can still be viewed?

The feature that allows part of an image to be made transparent, enabling objects or backgrounds behind that image to be viewed, is known as transparency. Transparency defines how see-through a particular part of an image is, allowing varying levels of visibility to whatever is situated behind it.

When transparency is applied, portions of an image can be made completely clear or semi-transparent, depending on the desired effect. This is commonly used in graphic design, web development, and image editing to create layering effects where the underlying elements are still visible through the transparent parts of the top image.

In contrast, while opacity relates to how opaque or solid an object is (with low opacity being more transparent and high opacity being more solid), it doesn't specifically designate areas of an image to be selectively transparent. Blend modes affect how colors mix between layers without explicitly creating transparency. An overlay typically refers to an additional layer placed on top without inherent transparency properties. Thus, transparency is the most accurate term to describe the capability of making parts of an image see-through.