Rituals are symbolic actions. In the context of LessWrong, it's significant that many rituals have some impact on your cognition, which makes them appropriate to be careful with. Nonetheless, some LessWrongers have worked to explore the space of ritual through a rationalist lens.
It's a bit tricky to define. The book Secular Wholeness notes:
There’s a hazy boundary between the words “ritual,” “habit,” and “custom.” I think the difference between a ritual act and a habitual one lies inawareness and assent. An act becomes a ritual for you when you perform it with conscious awareness of its symbolic and emotional meaning, and with willing assent to those meanings. Unless you act with both awareness and assent, your act is merely a habit (if it is unique to you) or a custom (if you share it with others).