A "host species" refers to an organism that provides a habitat or resources for another organism, often a parasite or symbiont. The host supplies essential nutrients, shelter, or support, allowing the other species to thrive. This relationship can be beneficial, neutral, or harmful, depending on the nature of the interaction. In ecological terms, host species play a crucial role in maintaining biodiversity and ecosystem balance. For example, in the case of coral reefs, corals serve as host species for various marine organisms, including zooxanthellae algae, which provide energy through photosynthesis while benefiting from the coral's protection.