Which word is the opposite of "periodical"?

The term "periodical" refers to something that occurs at regular intervals or is published regularly, such as a magazine or journal. The opposite of this would be something that happens infrequently or irregularly. "Sporadic" signifies events or occurrences that are scattered or not occurring in a regular pattern, thereby conveying the idea of unpredictability and irregularity. This makes "sporadic" the correct answer as it directly contrasts with the consistent timing implied by "periodical."

The other options do not capture this contrast effectively. "Subordinate" relates to rank or position within a hierarchy, with no inherent connection to the frequency of occurrence. "Ruse" refers to a trick or deception, which also does not relate to timing or regularity. "Alleviate" means to make something less severe or serious, which is unrelated to the concept of regularity or occurrence. Therefore, "sporadic" is the most fitting antonym for "periodical."