Of persons, their temperament, etc.: observant of, or having regard for, order, system, or method; methodical.

Of a person: acting with or observant of method or order; painstaking, meticulous.

That observes an established rule, principle, or course of action; acting or behaving in a uniform manner. In later use esp.: observing fixed times…

Of an action, pursuit, discourse, etc.: characterized by method or order; = methodical, adj. 1a; relating to method. Also (occasionally) of a person…

Arranged or conducted according to a system, plan, or organized method; involving or observing a system; (of a person) acting according to system…

= regular, adj. in various senses.

In extended use: systematic, methodical, meticulous.