The introduction to Shadows of Yog-Sothoth includes a "How to Use This Book" section that provides some insight into this matter, though, as we'll see, there are still complexities to be explored.
Shadows of Yog-Sothoth is divided into two main sections. The first part is a lengthy campaign of seven scenarios, from which the entire book takes its name. The second part is much shorter, and is a pair of bonus scenarios; unrelated to the first part, or to each other.
The first part of the book is meant to be played as a fully campaign, and those scenarios should be followed in the order that they are laid out in this book ...
As with The Traveller Adventure, we say a campaign as being defined as a collection of linked scenarios built around a common focus, in this case defeating the plans of "a sinister occult organization, led by the evil Lords of the Silver Twilight," as the introduction goes on to explain.
Speaking of the introduction, a later paragraph sheds some further light on the meaning of a campaign. After discussing how the campaign is structured – seven scenarios, each of which leads into the next one – it goes on to say:
If your particular mode of play is not suited to a series of linked adventures, such as this book consists of, the individual scenarios may certainly be played by themselves.
This sentence suggests that Call of Cthulhu has two "modes" of play: one based around individual scenarios and one based around "a series of linked adventures," which is to say, a campaign. Interestingly, the back cover of Shadows of Yog-Sothoth describes itself as a "scenario book for the CALL OF CTHULHU role-playing game. SHADOWS OF YOG-SOTHOTH is a seven-chapter tale of horror and suspense." Here, each integral scenario of the campaign is likened to a chapter, the whole – the campaign itself – being called a "tale of horror and suspense."
Chapter VII of the second edition of Call of Cthulhu bears the title "How to Play the Game." Here, we once again see that a "campaign" is defined as being made of several scenarios that
ought to be arranged like the branches of a tree. The players start out fooling around with the very tips and edges of the mythos, where there are dozens of myths, legends, clues, and adventures. As they gain knowledge and experience, the investigators will work their way inwards, where there are fewer happenings, of greater importance. At the center of the mythos reside the hideous Elder Gods in all their reality. The final goal of play may well be to save this planet and force the retreat of Cthulhu and other space beings. Becoming powerful to do that may take years.
Words like "scenarios," "adventures" and even "play" are used without precision. Are they all the same thing or are there fine distinctions between them? Where does a campaign fit into all this? That's why I find myself wondering whether anyone at Chaosium at the time had a clear sense of it themselves or whether we, in the present, are expecting a degree of clarity that no one at the time needed, let alone expected. Still, it's yet more grist for the mill as I delve more deeply into campaigns, their meaning, and how they were run during the first decade of the hobby.