What is referred to as the substance produced and expelled by a gland or body part?

The term "secretion" specifically refers to the process by which glands or body parts produce and release substances necessary for various bodily functions. These substances can include hormones, enzymes, and other biochemical compounds that play crucial roles in maintaining homeostasis, aiding in digestion, or regulating metabolic processes.

In contrast, the other terms have different meanings in the context of bodily functions. Excretion pertains to the removal of waste products from the body, absorption involves the uptake of nutrients and substances into the body's cells and bloodstream, and filtration relates to the process of separating substances based on size or other properties, often as part of kidney function. The concept of secretion is central to understanding many physiological processes and the functioning of endocrine and exocrine systems in the body.