noun which may take the genitive case particle 'no', na-adjective, noun
Used for actions or selections made without deliberate intent, often in formal or statistical contexts. The 'unintentional' nuance is less common than 'random'.
I bought a lottery ticket with randomly chosen numbers.
In the survey, we conducted a questionnaire with 1000 randomly selected people.
無作為 stresses 'randomness' or lack of intentional selection (as in statistics), whereas 無差別 stresses treating all targets equally, often in a harsher context like violence.
Compound of 無 (without) and 作為 (intentional action). The word 作為 itself is from Chinese, and 無作為 follows the pattern of negating a noun with 無.