From The Collaborative International Dictionary of English v.0.48:
Cowl \Cowl\ (koul), n. [AS. cuhle, cugle, cugele; cf. dial. G.
kogel, gugel, OF. coule, goule; all fr. LL. cuculla,
cucullus, fr. L. cucullus cap, hood; perh. akin to celare to
conceal, cella cell. Cf. Cucullate.]
1. A monk's hood; -- usually attached to the gown. The name
was also applied to the hood and garment together.
[1913 Webster]
What differ more, you cry, than crown and cowl?
--Pope.
[1913 Webster]
2. A cowl-shaped cap, commonly turning with the wind, used to
improve the draft of a chimney, ventilating shaft, etc.
[1913 Webster]
3. A wire cap for the smokestack of a locomotive.
[1913 Webster]
4. (aviation) a removable metal covering for an aircraft
engine, providing streamlining to minimize wind
resistance; -- also called cowling.
[PJC]
5. a covering for a chimney or other ventilating shaft
functioning to increase the draft.
[PJC]