INDIFFERENT

adj. apathetic, detached; unbiased, impartial ; unconcerned, disinterested; average; mediocre; unessential, unimportant

Apathy is a psychological term for a state of indifference — where an individual is unresponsive or "indifferent" to aspects of emotional, social, or physical life. Clinical apathy is considered to be at an elevated level, while a moderate level might be considered  depression, and an extreme level could be diagnosed as a dissociative disorder. The physical aspect of apathy associated with physical deterioration, muscle loss, and lack of energy is called lethargy — which has many  pathological causes as well.
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Adjective
1. marked by a lack of interest; "an apathetic audience"; "the universe is neither hostile nor friendly; it is simply indifferent"
(synonym) apathetic
(similar) uninterested
2. showing no care or concern in attitude or action; "indifferent to the sufferings of others"; "indifferent to her plea"
(similar) unconcerned
3. (usually followed by `to') unwilling or refusing to pay heed; "deaf to her warnings"
(synonym) deaf(p), indifferent(p)
(similar) heedless, thoughtless, unheeding
4. (often followed by `to') lacking importance; not mattering one way or the other; "whether you choose to do it or not is a matter that is quite immaterial (or indifferent)"; "what others think is altogether indifferent to him"
(synonym) immaterial
(similar) unimportant
5. fairly poor to not very good; "has an indifferent singing voice"; "has indifferent qualifications for the job"
(similar) inferior
6. having only a limited ability to react chemically; not active; "inert matter"; "an indifferent chemical in a reaction"
(synonym) inert, neutral
(similar) unreactive
(classification) chemistry, chemical science
7. marked by no especial liking or dislike or preference for one thing over another; "indifferent about which book you would give them"; "was indifferent to their acceptance or rejection of her invitation"
(similar) impartial, fair
8. characterized by a lack of partiality; "a properly indifferent jury"; "an unbiased account of her family problems"
(synonym) unbiased, unbiassed
(similar) impartial, fair
9. neither good nor bad; "an indifferent performance"; "a gifted painter but an indifferent actor"; "her work at the office is passable"; "a so-so golfer"; "feeling only so-so"; "prepared a tolerable dinner"; "a tolerable working knowledge of French"
(synonym) passable, so-so(p), tolerable
(similar) ordinary
10. neither too great nor too little; "a couple of indifferent hills to climb"
(similar) moderate

adv. indifferently, apathetically

adj. indifferent, apathetic; inert (Chemistry); rare, inactive

adj. indifferent, unconcerned, apathetic; uninterested, careless, unmindful; cold-hearted, thick-skinned, insensible, insouciant; listless, regardless, unheeding, unmoved

n. indifferent, apathetic person