The arrangement of elements to give the viewer the feeling that all the parts of the photograph
form a coherent whole.
Of all the principles of design, this one is the most difficult to illustrate, to understand in fact. As I use to tell my art students, "Unity means that you cannot take away any part without damaging the whole."
Like all the design principles, applying them to photography is a challenge because, unless you stage the photograph, what is before you is what you can photograph. Framing becomes critical and cropping can emphasize various elements.
In this photograph, made on South Uist, each element (the two flowering plants, fence post, the barn, the mountain and the sky) all flow together...each a crucial element. If you take your finger and block out one, the whole photograph falls out of harmony. Each is necessary. The flowers anchor the foreground, the fence post creates a bridge through the middle ground to the horizon, and the focal point, the barn buildings, give your eye somewhere to travel to. It is an inward and upward path. The mountains delineate the horizon line and lift your eye into the sky.
I made several studies of this scene before I felt I had re-created the experience of being at that particular place in that moment of time. The only cropping I did was to take a tiny bit off the sky so that horizon line didn't bisect the composition...something I never like particularly.
With your first response to the landscape, you may not be too aware of the individual elements and their relationship to the whole. Take your time and do several studies. One will stand out for you...one will contain the harmonious unity you seek. Again, it is with a conscious awareness that some level of mastery occurs. It is the difference between the thoughtful conceived image and a "snap shot".