A dream is a vision that a person sees during his sleep. The Arabic words for dream are ru'yaa (pl. ru'aa) and ḥulum (or ḥilm, pl. aḥlaam).
In the Qur'aan, the word ḥulum appears three times  - each time referring to mixed-up dreams, and the word ru'yaa appears seven times  - each time referring to truthful dreams.
In the Sunnah, "ru'yaa" and "ḥulum" are sometimes used interchangeably. More often, however, ru'yaa is used for a good dream and ḥulum for a bad one.
Imaam Ibn ul-Manẓoor رحمه الله said: "Ru'yaa and ḥulum is what a person sees during his sleep. But ru'yaa is mostly applied to what he sees of good and pleasant things, and ḥulum is mostly applied to what he sees of evil or bad things. And either of the two words may be used in the place of the other."