Landmark works, sometimes called seminal or classic studies, are articles that initially presented an idea of great importance or influence within a particular discipline. These are the articles that are referred to time and time again in the research, so you are likely to see these sources frequently cited in other journal articles, books, dissertations, etc.
Identifying a landmark article relies heavily on your own thoroughness in examining and synthesizing the existing literature. Typically, there will not be any explicit labels placed on articles, identifying them as "landmark" or "seminal". Rather, you will begin to see the same authors or articles cited frequently.
It is important to keep in mind that seminal studies may have been published quite some time ago.
Limiting a database search to only the past 5 years, for example, may exclude landmark studies from your results.
To avoid overlooking pivotal research that may have occurred in years past, it is recommended that you do not use a date limiter when searching for landmark articles.