"Karim looked around but could not find the key." the word, "around" is a/an:

Around:
- In a position or direction surrounding, or in a direction going along the edge of or from one part to another.
- বাংলা অর্থ: চতুর্দিকে; এখানে-সেখানে

Around can be used in the following ways:

- as a preposition (followed by a noun): We walked around the old town.
- as an adverb (without a following noun): She turned around and smiled at me. (After the verb ‘to be): Don’t discuss this when the children are around

যেহেতু এই বাক্যে এর পরে কোন noun নাই, সুতরাং এটি এক্ষেত্রে একটি adverb হিসেবে ব্যবহৃত হচ্ছে।

Source: Macmillan Dictionary, Grammar reference.