This is the 6311th most frequent Japanese word.
コ
"コ" is a katakana character used in Japanese to represent the "ko" sound.
The word 'コ' is the initial part of 'コーヒー', which means 'coffee'. It represents the phonetic importation of the foreign word 'coffee'.
コーヒーを毎朝飲みます。
I drink coffee every morning.
The word 'コ' appears within the onomatopoeic expression 'コロコロ', wherein it mimics the rolling and playful sounds.
猫がコロコロと遊んでいます。
The cat is playing and rolling around.
The word 'コ' is part of 'この', which is a determiner used to specify the proximity of 'the book' to the speaker.