The definition of aesthetic (adjective), in the most basic, colloquial sense, is "pertaining to the appreciation of beauty or good taste." The corresponding noun is "aesthetics," which means "the study of the appreciation of beauty or how we perceive beauty." This study is considered important enough to constitute a branch of philosophy.
But this article restricts itself to helping you understand what "aesthetic" means in the context of landscape design, where the term is often used colloquially (that is, not in the deep, philosophical sense, but, rather, casually). One might say, for example: "That homeowner's yard is aesthetically challenged," meaning that one does not like the way it looks.
Landscape design is concerned both with aesthetic and functional elements of landscaping. While the former is ultimately subjective (there is no accounting for tastes, as the old expression goes), professional landscape designers are, in fact, guided by some basic rules that help them to create aesthetically pleasing designs in their clients' yards.
Where landscape design is concerned with aesthetics, the terminology used is somewhat similar to that employed in the art world. For example, browse some of the resources offered in the following article:
You will see that, discussed in those resources, are topics such as color theory, form and texture, the role of focal points, the definition of terms such as "line" and "scale," etc.
One of the great aesthetic debates the last few centuries between gardeners has been one focused on overall style, namely, between the styles of the informal and formal garden design schools. The latter prefers symmetry, tight structure and orderliness. A classic look in a formal garden design is a hedge, neatly manicured, consisting of English boxwood. Such a hedge may serve as a divider between planting areas, a firm indicator of demarcation.
By contrast, those whose aesthetic tastes lean toward informal garden design may be drawn to cottage gardens, marked by a wild riot of color and a disdain for obvious organization.
While much study has gone into comparing and contrasting the formal and informal styles of landscape design, there is also much that is subjective in what we do or do not find aesthetically pleasing in landscaping. For example, some critics will decry the use of certain plants in a yard simply on the basis that they are very commonly used. Thus you will sometimes hear that such and such a plant is "overused." This charge is misleading for beginners and really no more than a personal opinion, as argued in the following two articles that deal with plants that are allegedly overused:
Addendum: Terms Related to "Aesthetic" to Help You Remember the Definition
While the connection of the first term (which means a licensed skincare specialist) to "aesthetic" is straightforward enough, it will not be so clear to many how the words "aesthetic" and "anesthetic" are related. The prefix an- means "without" or "not" in Greek. When you are anesthetized, your ability to perceive or to feel is (intentionally) impaired.
Also note than an alternate spelling for the term is "esthetic."