What are secondary qualities in John Locke’s philosophy?

In John Locke’s philosophy, as detailed in his An Essay Concerning Human Understanding, secondary qualities are distinguished from primary qualities based on how they relate to physical objects.123

Secondary qualities include sensations such as colour, taste, and smell. Unlike primary qualities (such as size, shape, or solidity), which are considered real properties that exist within an object, secondary qualities do not resemble any actual property in the object itself.123

Instead, Locke argues that secondary qualities are the product of the power an object possesses to cause specific ideas in the mind of the perceiver. These ideas arise through the interaction between the object's primary qualities, external conditions like light, and the perceiver’s sense organs. Consequently, while primary qualities represent the object as it is, secondary qualities are merely effects produced within the mind.123