General term for something that acts as an intermediary or means of transmission, such as a medium for communication, a carrier of information, or an agent that facilitates a process.
Language is a medium of communication.
Air acts as a medium for sound.
Medical/epidemiological use: an organism or object that transmits a pathogen. 媒介物 can refer to a biological vector like a mosquito, or an inanimate object (fomite) that carries infectious agents.
Mosquitoes are vectors of malaria.
Contaminated towels became a fomite for the infection.
媒介 is the act or process of mediation/transmission, while 媒介物 is the concrete thing that does the mediating.
Compound of 媒介 (mediation, intermediary) and 物 (thing, object). The exact historical derivation is uncertain, but the word follows a standard Sino-Japanese compounding pattern.