El siglo, un siglo. Listening file: To listen to the Spanish pronunciation of "siglo", you can either click here or here:... . siglo (el). English translation: Century. Synonyms (According to WordReference.com): la centuria , el centenario.

“La centuria” is not very colloquial and, about “el centenario” (centenary, hundredth anniversary), we use this word in sentences such as “El centenario de la muerte de Gandhi.” (la muerte = the death)

This site is also mentioning, as synonyms of “el siglo”, the Spanish words “(el) mundo and (la) sociedad", where “mundo” means “world” and “sociedad” means “society”.

Gender: “siglo” is a masculine noun (substantive), so any article or adjective connected to this noun will be masculine. For example: “el siglo”, “este siglo” “el siglo pasado”, “un siglo nuevo”, where “el”, “este”, “un” are masculine determinants, and “pasado” and “nuevo” are masculine adjectives.

This word follows the general rule: Spanish nouns ending in –o, are masculine and, as this words ends in vowel, its plural form will be “siglos” (by adding “–s”).

Etymology: from Latin “saeculum”.

Definitions:
- Rae.es includes the following definitions: (English translations (in blue) are literal, word by word, so you can recognize Spanish words and their English meanings)

1. Periodo de cien años. (“Period of one hundred years.”)

2. Tiempo en el que alguien vivió o algo sucedió o se inventó. (“Time when someone lived or something happened or was invented.”)
Example: “El siglo de la tecnología.”

3. Cuando queremos expresar la idea de “mucho tiempo”.. (“When we want to express the idea of “a lot of time”.). Example: “Hace siglos que no voy al cine.” (A lot of time since I went to the movies.”)

Uses and expressions:
The Rae.es is also including two locutions: the first one is very colloquial and everybody uses it, but the second one... I have never heard about it before!

1. “por los siglos de los siglos”. It means “for ever and ever”

2. “por el siglo de mi padre” or “por el siglo de mi madre”. This is used, according to the RAE, to promise something, invoking the memory of someone you loved a lot and who is dead.

WordReference.com includes the following:
3. “Mi madre es de otro siglo.”. This means that my mother is very old fashioned, but talking about her opinion, not about having no dress sense!

4. “Siglo” is also used as “Siglo de Oro” (“Golden age”) or “Siglo de las Luces” (Age of Enlightenment”).

Sources:
Wordreference.com
Real Academia de la Lengua Española

Interested in Spanish vocabulary? If so, the book Practice Makes Perfect: Spanish Vocabulary can be helpful.