The phrase "clavicular head" refers to a specific part or section of a muscle called the clavicle, which is the bone in the front of your shoulder. Full definition
Strangely, even though larger muscles tend to be more pennate than smaller ones, the smaller clavicular head of the pectoralis major appears to be more highly pennated than the much larger sternocostal head. (strengthandconditioningresearch.com)
Glass & Armstrong (1997) compared the 15 degrees below horizontal decline bench press to the 30 degrees incline bench press and found no difference in pectoralis major clavicular head muscle activity. (strengthandconditioningresearch.com)
They reported a main difference of clavicular head muscle activity between 100 % (superior) and 200 % grip width. (strengthandconditioningresearch.com)