Definition 1 (what is hard rock):
Hard rock is a genre of rock music with special emphasis on distorted electric guitars. It is often described as "aggressive", "heavy" and "anthemic". Its harmonic structure is usually based on power chords played over a blues scale. Hard rock originated in late 1966 with the first recordings by The Jimi Hendrix Experience and Cream. Its stylistic origins lie primarily in the garage rock of the first half of the 1960s.
Definition 2 (what isn't hard rock):
For the purposes of the list, artists from the following genres were not considered for this list, even though they may suit the definition given above: garage rock (e.g. The Sonics, The Kinks, Monks), heavy metal/proto-metal (e.g. Black Sabbath, Budgie, Sir Lord Baltimore), punk/proto-punk (e.g. The Stooges, MC5, New York Dolls), alternative rock/grunge (e.g. Pearl Jam, Stone Temple Pilots, Living Colour) and stoner rock (e.g. Queens of the Stone Age, Kyuss, Monster Magnet). The reason for this is that punk, metal and alt-rock already have their own lists, and thus it would be redundant to list them again. Also, they represent different branches of the rock tree with different stylistic features (for example, they're often not as clearly rooted in blues). Garage rock on the other hand is a sort of proto-'hard rock' with a more amateurish approach. Stoner rock is an almagam of influences from different styles (mainly hard rock, metal and alt-rock) that has really developed into something singular.
Definition 3 (what is hard rock anyway):
Even though metal artists were not considered for this list, glam metal was treated as a subgenre of hard rock as it's not really metal anyway. Other than that, artists from the following genres were often allowed if they rocked hard enough: blues rock, AOR, arena rock, southern rock, boogie rock, progressive rock, glam rock, psychedelic rock. As a rule of thumb, artists should have rocked harder than The Rolling Stones but not as hard as, say, Metallica. If an artist only played hard rock for a certain period of their career or only included some hard rock songs on their albums, sandwiched between softer material, they were allowed if they recorded at least an album's worth of hard rock material.