Definition

There is no definition for "dream journal" in the dictionary or from dictionary.com, but it essentially is a place where someone records their dreams, whether on paper or electronically.

I decided to look up the definitions of "dream" and "journal/diary" separately.
The most interesting thing for dream I found were the synonyms.

Dream : –noun

1. a succession of images, thoughts, or emotions passing through the mind during sleep.

2. the sleeping state in which this occurs.

3. an object seen in a dream.

4. an involuntary vision occurring to a person when awake.

5. a vision voluntarily indulged in while awake; daydream;  reverie.

6. an aspiration; goal; aim:

7. a wild or vain fancy.

8. something of an unreal beauty, charm, or excellence.

—Synonyms
1. Dream, nightmare,  and vision  refer to the kinds of mental images that form during sleep. Dream  is the general term for any such succession of images. A nightmare  is a dream that brings fear or anxiety: frightened by a nightmare. Vision  refers to a series of images of unusual vividness, clarity, order, and significance, sometimes seen in a dream. 


Diary :

–noun, plural -ries.

1. a daily record, usually private, especially of the writer's own experiences, observations, feelings,attitudes, etc.

2. a book for keeping such a record.

3. a book or pad containing pages marked and arranged in calendar order, in which to noteappointments and the like.

—Synonyms
1, 2.  journal, daybook, log, chronicle