Yes it is a form of meditation because you are not aware of the conscious world around you, time and distractions don't exist. The only difference is with daydreaming, you are not 100% controlling your mind, it is sort of like you are letting the stories in your mind happen. For most people the main thing lacking there is your focus, some people focus on breathing first to put to get them to a relaxed state, some people focus on an image, some on nothing but empty space and some people focus on their mid point being a ball of energy. You can though focus on and control a detailed story, focussing on making the colours bright and vivid in your mind, feeling the wind, smelling the scents of your story. That is where your focus comes from and you are controlling the story rather than allowing it to happen.
However as Odalibuc rightly said, people with conditions such as ADHD will find it difficult to concentrate on a sustained focus for a period of time, but there are herbal remedies that you can take to help calm your mind such as lime blossom or valerian. I would recommend meditation to anyone, even 10-15 minutes a day can help you feel more balanced in your life.
I've always controlled my daydreams, I don't let them run off like what happens when I am dreaming when I'm asleep. When I'm sleep dreaming, I don't really have very much control over where the dream goes, I'm not very good a lucid dreaming... Yet. But my daydreams are controlled enough to the point that I can feel what I want to feel, smell what I want to smell, my body reacts to what is happening in the daydream, etc. Now ask me to visualize a red square in a black room, and hold it there... Ack. Lol.