The oxidized (aged) form of THC. CBN is a weaker version of THC.
When Δ9-THC oxidizes, which happens as cannabis ages, it degrades into CBN. CBN has about 25% the intoxicating ability as Δ9-THC. Many have attributed sedative effects to CBN, but these claims are not substantiated by scientific literature.
Summary from Wikipedia:
Cannabinol (CBN) is a mildly psychoactive cannabinoid found only in trace amounts in Cannabis, and is mostly found in aged Cannabis. Pharmacologically relevant quantities are formed as a metabolite of tetrahydrocannabinol (THC). CBN acts as a partial agonist at the CB1 receptors, but has a higher affinity to CB2 receptors; however, it has lower affinities relative to THC. Degraded or oxidized cannabis products, such as low-quality baled cannabis and traditionally produced hashish, are high in CBN.
Unlike other cannabinoids, CBN does not stem directly from cannabigerol (CBG) or cannabigerolic acid (CBGA), but rather is the degraded product of tetrahydrocannabinolic acid (THCA). If cannabis is exposed to air or ultraviolet light (for example, in sunlight) for a prolonged period of time, THCA will convert to cannabinolic acid (CBNA). CBN is then formed by decarboxylation of CBNA.Read more