Plagiarism is defined as:

the use of the work of others without acknowledgement.

This covers not just using words, but also, for example:

  • concepts
  • ideas
  • data
  • designs
  • images
  • computer programmes
  • music

Note in particular that it refers to ideas, not just to words, so even if you express someone else’s ideas in your own words, the source of the idea must still be acknowledged.