Inverse time just means the time to trip is the inverse of the rate of rise in current; as the current goes up, the trip time goes down. All "Thermal Mag" (aka T-M) circuit breakers are inverse time. The NEC uses this term to differentiate from a magnetic-only breaker, which is referred to as "Instantaneous Trip" (I-T) in the NEC. There are other types of circuit breakers that have more specific uses as well, but because they are so specific, the NEC doesn't need to mention them.
ITE invented the "Inverse Time Element" concept back in the first part of the 1900s, hence their name. But the concept is generic. Back then you could achieve an inverse time trip curve with different technologies, for example a "Dash Pot Trip Element" used hydraulic fluid forced through an orifice that changed how fast it flowed based on the viscosity changing with temperature. What ITE invented was a single electro-mechanical thermal element that did it reliably and repeatedly. When their patents ran out, everyone else adopted it.
