Adharma is the opposite of dharma. This is a compound formed from the word dharma and the negative particle na that changes to a to form the substantive adharma, meaning ‘not dharma’. Dharma is defined in a glossary listing.
Technical: The substantive adharma is a sub-type of tatpuruṣa compound called nañtatpuruṣa. The coded negative particle nañ is just na, the code ñ having no significance in compound formation. Thus nañ + dharma = na + dharma = a+ dharma = adharma. The initial consonant of na, n is dropped when it is followed by a term beginning with a consonant, thus forming adharma. This negative compounds are similar to typical – atypical, tenable – untenable, possible – impossible in English. Just like the a, un and im imply the opposite meaning of the following word fragment, in Saṁskṛta, the initial syllable will become a and an – dharma – adharma, karma – akarma, anta – ananta (the last example is that of a negative compound formed with a substantive beginning with a vowel.