An engram is a proposed physical substrate of a memory in the brain—the neural changes that encode and store a learned experience, so that recall or behavioral expression of that memory can occur later. In short, it’s the brain’s memory trace. Key points to understand about engrams:

• Biological basis: Engrams are thought to comprise specific patterns of neurons and synaptic connections that are activated together to form a memory, along with the coordinating activity in neural circuits (e.g., assemblies of cells and their synaptic weights) and related brain rhythms. This framing emphasizes that memories have objective, measurable brain states beyond subjective recollection.
• Historical origin: The term was coined by Richard Semon in the early 20th century to describe the enduring modifications in neural tissue produced by a stimulus, which underlie memory. Since then, research has evolved to identify candidate cellular and molecular substrates of engrams.
• Modern view: Engrams are not a single isolated site but can involve distributed networks across multiple brain regions. Modern techniques allow scientists to label, manipulate, and observe these memory-related ensembles, providing insight into how memories are formed, stored, and retrieved.

If you’d like, I can tailor this with examples from different memory types (like fear conditioning vs. object recognition), or explain how scientists experimentally identify and manipulate engrams in animal models.