(topology) A simplicial complex where every collection of pairwise adjacent vertices spans a simplex.

(Britain, Ireland) A day on which a registered charity raises money, usually by selling small lapel flags. / (software) A day designated for changing to an incompatible system.

(Australia) September 3, for the first flying of the Australian flag in 1901, Australian National Flag Day.

(transitive, idiomatic) Use a flag or similar kind of signal to get someone's attention.

(nautical) A black and yellow flag (signal flag LIMA) formerly hoisted by a ship to show that it has contagious or infectious disease aboard, or by a ship that has been quarantined. This use was listed in Brown’s 1916 code of signals, but this use is no longer listed in contemporary editions of the International Code of Signals.

(figuratively) A cue, warning, or alert; a sign or signal that something is wrong. / (idiomatic, US) red rag

A perennial wetland plant, Acorus calamus, with an aromatic medicinal root, formerly used to strew floors.

One who carries a flag, especially at a ceremony. / One who openly promotes an idea or value and becomes symbolic for it.

(motor racing) A black-and-white chequered flag which signifies the end of a race. / (motor racing) A victory in a race.

The act of burning a flag, especially the flag of a nation as an act of protest at that nation's activities.