Specimen plants are plants grown by themselves, usually, in a lawn or garden, for ornamental effect, rather than being massed with others as are bedding plants, hedge plants, or edging plants. Specimen plants can thus serve as focal points in a landscape design.
It is easy to remember what "specimen" plants are if you learn the Latin root that the word is based upon. The Latin root, spec- means "to look at." Consequently, a specimen plant is one you would single out in your landscape as being particularly worth looking at and thereby deserving of "center stage" in your yard.
Often this will be a tree, but it could also be a shrub, perennial, or other plant if it is sufficiently large, especially showy, cuts a bold figure, or strikes the eye as being quite unusual.
An example of a specimen plant is a flowering tree that has a prominent spot reserved for it on a lawn. What type of tree should you use? Well, that is a highly personal choice. For example, if you have a favorite color, you might make your selection from a number of candidates whose flowers sport that particular color. But it could be any visual attribute that you, yourself attach value to.
I happen to use a golden chain tree in this manner in my own yard. I value the look of its abundant and long flower racemes. But again, it is a matter of personal choice. A possible objection here is that golden chain trees do not bloom long enough to warrant their use as specimen plants. This is a serious objection, because they do not have any other noteworthy qualities that can pick up the slack during periods when they are not flowering (for example, colorful bark or unusual leaves).
But my own opinion is that their brilliance while in bloom trumps the brevity of the blooming period and the fact that they are, sadly, one-trick ponies.
Some people would make pleasing plant form the criterion they use for selecting a specimen plant, rather than pretty flowers. Their argument is that an interesting branching structure, for example, can be appreciated year-round, rather than for just the short period during which a tree or shrub blooms.
It is easier to make that case in the context of landscaping for small spaces, where space constraints may limit you to one specimen plant.
Plants with interesting forms include:
Likewise, some would argue that trees or large shrubs with evergreen foliage are suited to be used as specimen plants, as long as they are striking enough to hold one's attention. Their great strength in this regard is that they keep their leaves year-round and so always look nice. Examples include:
Ideally (and if your landscape is large enough), you would include at least one plant you regard as a specimen plant in your landscaping for each of the four seasons. That way, you would have something to look forward to throughout the year without having to put all your eggs in one basket. This would allow you to use a golden chain tree as a specimen plant for spring, for example, knowing that you will have other specimen plants to rely on to keep your landscape interesting during the other three seasons of the year.