This is the 1537th most frequent Spanish word.
reino
"Reino" means "kingdom."
Here, 'reino' refers to a political or historical concept of a 'kingdom'.
El reino antiguo prosperó bajo el mandato de un rey sabio.
The ancient kingdom thrived under the rule of a wise king.
In this usage, 'reino' metaphorically stands for a domain or field, specifically in a conceptual or abstract sense.
En el reino de las ideas, toda imaginación es bienvenida.
In the realm of ideas, all imagination is welcome.
Here, 'reino' is used metaphorically to represent a period of dominance or prevalence of an idea, in this case, peace.