century
The Italian word 'secolo' is used to describe a period of 100 years. It is often used in historical contexts to denote a particular century, such as 'il ventesimo secolo' for the twentieth century. It can also be used to express a long period of time, for example, 'un secolo fa' means 'a century ago'.
That castle was built in the 15th century.
This sentence is using 'secolo' to refer to a specific century, the 15th, in which an event occurred, specifically, when a castle was built.
The Gothic style was popular during the 14th century.
In this case, 'secolo' is used to define a specific time period, the 14th century, during which the Gothic style was popular.
The last century was marked by numerous changes.
Here 'secolo' is referring to the last century in a more general context, talking about changes that happened during that time period.