The term 'Senior One' is used to refer to the ELDEST or most experienced member of a group. The crossword solutions for this term are 'ELDEST' (6 letters) and 'DOYEN' (5 letters). 'ELDEST' is an adjective that means the oldest or most senior member of a group, while 'DOYEN' is a noun that refers to a person who is the most experienced or respected in a particular field. Both terms can be used to describe the same individual, but 'DOYEN' implies a higher level of expertise or authority.