Looking up...
By oneself; without anyone else present
She prefers to read alone in her room.
She enjoys solitude and often reads alone in her room.
He felt alone in the crowd.
Despite being surrounded by people, he felt isolated and alone.
Can express physical solitude or emotional isolation.
Without any other similar or comparable thing; unique
This painting is alone in its style.
No other painting has the same unique style as this one.
Note that 'alone' can describe both physical solitude (no one else present) and emotional isolation (feeling lonely even in a crowd).
Use 'alone' to describe being by oneself, and 'lonely' to describe the feeling of sadness from being alone.
From Middle English 'alon', from Old English 'ān' (one) + 'lone' (alone). Related to Old Norse 'allinn' and German 'allein'.
Can describe physical presence or emotional state. Often used to emphasize solitude or uniqueness.