Which word means to press down tightly?

The term that means to press down tightly is "tamp." This word is often used in contexts where materials, such as soil or coffee grounds, need to be densely packed. For example, tamping down soil ensures that it is firm and stable, preventing any air pockets that might lead to instability. In the context of making espresso, tamping involves pressing the coffee grounds firmly into the portafilter to encourage even extraction during brewing.

While "compact," "squeeze," and "flatten" also convey ideas related to pressing or condensing, they lack the specific connotation of tightly packing or firmly pressing down that "tamp" provides. "Compact" implies that something is made smaller in overall volume, but doesn't necessarily convey the action of pressing down tightly in the same controlled manner. "Squeeze" suggests applying pressure but may not result in an even or uniform packing. "Flatten" focuses more on reducing height or making something level rather than the act of compressing it tightly. Thus, "tamp" is the most precise term for the action described in the question.