flagship  (n.) the main or most successful product in a range 旗舰，同类产品中佼佼者

[例]  Their cars are flagships among all the products in this city.
　
flat  (adj.) commercially inactive; sluggish 萧条的

[例]  The American market was flat in 1995.

flawed (adj.) having defects有瑕疵的

[例]  I am writing to apologize for our flawed samples.

flexitime  (n.) (also flextime) flexible system of working in which workers can choose what time to start and finish (see also core time)弹性上班制

[例]  With the flexitime system you can choose your start and finish time.

flier  (n.) a promotional leaflet 广告传单

[例]  Fliers of this new type of computer are handed out in all the streets.