Batten is a long, narrow strip of wood used to strengthen, fasten, or support something. Carpenters use battens to hold boards together, secure edges, or create frames. A barn door might have battens running across its back to keep the vertical planks from sagging or splitting apart. Theater crews attach battens to the backs of painted scenery to keep the canvas flat and rigid.

On sailing ships, battens serve a different but related purpose: they're thin, flexible strips inserted into pockets in the sail to help it keep its shape and catch the wind more efficiently. Without battens, a sail might flutter uselessly or fold in ways that waste the wind's power.

The most famous use of the word appears in the old nautical command “batten down the hatches,” which means to prepare for trouble by securing everything tightly. Sailors would cover the ship's hatch openings with tarpaulins and nail wooden battens over them to keep water out during storms. Today people use this phrase when getting ready for any difficult situation: a family might batten down the hatches before a blizzard by stocking up on food and firewood, or a student might do the same before final exams by clearing their schedule and gathering study materials.

As a verb, to batten can mean to fasten something with battens, or to make something more secure.