To lengthen is to make something longer, either physically or in time. During the spring and early summer, the days lengthen, as sundown comes later and later.

Mascara lengthens your eyelashes, and letting out your hem lengthens your jeans. As time goes by, your years of experience lengthen, and if you don't cut your hair or trim your beard, those lengthen as well. Lengthen dates from the 14th century — earlier, the verb was length, from a Germanic root that's also the basis of "long."

Definitions of lengthen
  1. verb
    make longer
    Lengthen this skirt, please”
    synonyms: elongate
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    make shorter than originally intended; reduce or retrench in length or duration
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    draw out, extend, prolong, protract
    lengthen in time; cause to be or last longer
    elongate, stretch
    make long or longer by pulling and stretching
    prolong the time allowed for payment of
    temporise, temporize
    draw out a discussion or process in order to gain time
    spin, spin out
    prolong or extend
    shoetree, tree
    stretch (a shoe) on a shoetree
    type of:
    make bigger or more
  2. verb
    become long or longer
    “In Spring, the days lengthen
    synonyms: elongate
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    become short or shorter
    type of:
    become larger, greater, or bigger; expand or gain
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