From Halopedia, the Halo wiki
Zen is a mental and spiritual type of thought and practice, that was practiced in various ways by Spartan-IIs to improve their skills in the 26th century.
Usage and examples
Linda-058 is "icy cold state of Zen no-thought" whenever she is sniping in combat. Kelly-087 practices Zen arts on a daily basis, including Zen twitch response drills to keep her reflexes sharp. Naomi-010 thought she sounded a bit zen when she said "You have to know how much gas to pour on a fire" to Vasily Beloi, a thing Beloi later thought about when Serin Osman drip fed the Servants of the Abiding Truth supplies.
UNSC Artificial Intelligence constructs are fascinated by Zen philosophy because of their natural creativity and limited life span. Dr. Catherine Halsey, designer of many of these AIs, is sometimes able to manipulate them by presenting them with Zen philosophies.
List of appearances