sporadic
Happening only once in a while, without a regular pattern.
Sporadic means happening irregularly, with no predictable pattern. When something is sporadic, it occurs occasionally and unpredictably, like scattered raindrops rather than a steady downpour.
A student might make sporadic efforts to practice piano, playing intensely for a few days, then not touching the keys for weeks. Wild thunderstorms might make sporadic appearances throughout summer, impossible to predict in advance. A town might experience sporadic power outages during a storm, with electricity flickering on and off at random intervals.
The word captures that sense of unpredictability and inconsistency. Sporadic is the opposite of regular, steady, or constant. If you exercise sporadically, you're not following any real schedule: you might go for a run three days in a row, then not exercise again for two weeks. If a friend sends you sporadic text messages, they pop up unexpectedly without any pattern you can count on.
Scientists use this word frequently. They might track sporadic meteor showers that appear without warning, or study diseases that show up in sporadic cases rather than predictable outbreaks. The key is always the same: sporadic events happen now and then, but you never quite know when the next one will arrive.