What does the author mean by term 'Mutation'?

Very early in the study of genetics biologists began to pay particular attention to the aberrations now known as mutations; this investigation was initiated by de Vries at the turn of the century and brilliantly developed by Morgan and his school from 1910 on. A gene, by definition, is a living particle that reproduces itself, and nearly always it does so exactly. But occasionally, probably under the influence of some outside agent, it forms an imperfect copy of itself. It this altered gene is able to reproduce itself in its new form, the results is a mutation, which is evidenced in permanent change in some trait of the organism.