Final answer:
The term 'Roger' is an old radio communication term used to indicate agreement or compliance, representing the letter 'R' for 'received'.
Step-by-step explanation:
The old term used in radio communications to indicate agreement or compliance is 'Roger'. Historically, 'Roger' was used because it represents the letter 'R' for 'received' in the phonetic alphabet. Prior to 1957 when the current phonetic alphabet was standardized, 'Roger' was used in radio voice procedure. Afterward, the word 'Roger' was informally maintained to indicate that a message was received and will be complied with, despite 'Romeo' being the official phonetic term for ‘R’.