1. sad because one has no friends or company
2. without companions; solitary
3. remote and unfrequented by people

Examples

• She felt lonely after moving to a new city.
• The old man lived a lonely life in the mountains.
• I get lonely when my roommate travels for work.
• The lighthouse stood on a lonely stretch of coast.
• He took a lonely walk through the empty park.
• The lonely road stretched for miles without any houses.
• After her friends graduated, she felt lonely at school.