Which kind of part of speech is the underlined word in the sentence below? The train arrivedyesterdayafternoon.

In the sentence "The train arrived yesterday afternoon," the underlined word "yesterday" is an adverb.

• Adverb: 
 Adverbs that change or qualify the meaning of a sentence by telling us when things happen are defined as adverbs of time.
- যে adverb কোনকিছু কখন/কোন সময়ে ঘটেছে তা বর্ণনা/উল্লেখ করে তাকে Adverb of time বলে।
- এই ধরণের adverb গুলো ক্রিয়া সম্পন্ন হওয়ার সময় বুঝায় অর্থা , 'when?' বা 'কখন?'  উত্তর দেয়।
- এটা sentence এর শুরুতে অথবা শেষে ব্যবহৃত হয় ।
- Adverbs of time: Yesterday, today, afterward, always, immediately, last month, now, soon, etc. are common.

Source: Cambridge Dictionary