Today's word is BELLUM, which is a second-declension neuter noun.
Latin meaning and usage: The basic meaning of the Latin word is "war" or "warfare." Not surprisingly, the Romans had a lot to say about war and many idiomatic expressions using this word. For the various prepositions and verbs commonly used together with this noun, take a look through the Lewis & Short dictionary entry.
Latin word formation: The crucial thing to understand about this word is that it was originally duellum. The intial du- changed to b- as similarly in the word bis, derived from duo. You need to be careful not to confuse this noun with the adjective bellus, which is a diminutive ultimately derived from the adjective bonus. They may look the same - bellum and bellus - but they have quite different origins! The noun bellum is a highly productive root in Latin, with nouns like bellator and belligerator, and adjectives like bellicosus and bellipotens, and compound verbs like debello and rebello.
English cognates and derivatives: You can see this Latin word in the English "antebellum," meaning "before the war." A "belligerent" is someone who wages war, and someone who is "bellicose" is warlike. Note also that we use the word "duel" in English, as in the old Latin duellum. The Harry Potter novels have given new life to the name "Bellatrix," a female warrior. There is also "rebel," from the compound Latin verb rebellare, meaning to "wage war against, rebel."
Here are some examples of today's word in Latin sayings and proverbs; for more examples, see the page at the Scala Sapientiae, which also contains notes on some of the proverbs cited below:
Habet et bellum suas leges.
Proelio victus, non bello.
Ex bello pax.
Si vis pacem, para bellum.
Pax quaeritur bello.
Paratur pax bello.
Bellum pacis est causa.
Nulla salus bello.
In bello nec primus nec ultimus esto.
Pace belloque fidelis.
Paritur pax bello.
Alia ex aliis nascentur bella.
Solum bellum gignit pacem.
Post bellum auxilium.
Pacem cum inimicis, bellum cum vitiis.
Dulce haud expertis est bellum.
Saepe sub nomine pacis bellum latet.
Pax, pax! clamatur; sed pax per bella paratur.
Bellum se ipsum alet.
Non licet bis in bello peccare.