Final answer:
The significant delay between the introduction of photographic self-portraiture by Robert Cornelius in 1839 and the widespread use of the word "selfie" in the 2000s exemplifies 'cultural lag'.
Step-by-step explanation:
The gap between the first instance of a self-portrait taken by a camera and the later widespread use of the term "selfie" is an example of cultural lag. This phenomenon occurs when there is a slow adoption of social behaviors or languages even after new technology or science makes it possible. In the case of the self-portrait taken by Robert Cornelius in 1839 and the word "selfie" only gaining popularity in the 2000s, we witness a significant delay. The term became so prevalent that it was chosen as the Oxford English Dictionary's word of the year in 2013, reflecting its cultural impact long after the technological means for self-portraiture was possible.