This is the 4719th most frequent Arabic word.
الاحد
"الاحد" means "Sunday" in English.
In this sentence, 'الأحد' (Sunday) is used to indicate the name of a specific day of the week.
ذهبَ أحمد إلى المدرسة يوم الأحد.
Ahmed went to school on Sunday.
Here, 'أحد' (one person) is used within a negative construction to denote exclusivity.
في ذلك الوقت لم يكن في العالم إلا أحد.
At that time, there was no one in the world but one person.
In this case, 'أحد' (one) notes an indefinite individual, demonstrating its usage as an indefinite pronoun.