Neurasthenia (from the Ancient Greek νεῦρον neuron "nerve" and ἀσθενής asthenés "weak") is a term that was first used at least as early as 1829 for a mechanical weakness of the nerves and became a major diagnosis in North America during the late nineteenth and early twentieth centuries after neurologist George Miller Beard reintroduced the concept in 1869.

As a psychopathological term, the first to publish on neurasthenia was Michigan alienist E. H. Van Deusen of the Kalamazoo asylum in 1869, followed a few months later by New York neurologist George Beard, also in 1869, to denote a condition with symptoms of fatigue, anxiety, headache, heart palpitations, high blood pressure, neuralgia, and depressed mood. Van Deusen associated the condition with farm wives made sick by isolation and a lack of engaging activity, while Beard connected the condition to busy society women and overworked businessmen.

Neurasthenia was a diagnosis in the World Health Organization's ICD-10, but is no more diagnosed in ICD-11, marked as deprecated. It also is no longer included as a diagnosis in the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders. The condition is, however, described in the Chinese Society of Psychiatry's Chinese Classification of Mental Disorders.

Americans were said to be particularly prone to neurasthenia, which resulted in the nickname "Americanitis" (popularized by William James). Another, rarely used, term for neurasthenia is nervosism.