Someone who can function in spite of his alcoholism (masking his disease). Most functioning alcoholics have maintenance levels of alcohol in their systems. They wake up in the morning, begin drinking up to that level and then maintain it.
A person who appears to function in society when under the influence of alcohol. Upon closer inspection is found to have established a network of enablers who facilitate the apearance of functioning; i.e: drinking partner who is also the workplace manager, relative who transports to work, etc. Tongue in cheek definition: Functional Alcoholic-"the person's drinking doesn't bother me, yet."