Yes, solo is a Scrabble word!

Adverb

1. Without anybody else or anything else

Verb

1. Fly alone, without a co-pilot or passengers

2. Perform a piece written for a single instrument

Adjective

1. Composed or performed by a single voice or instrument

Noun

1. Any activity that is performed alone without assistance

2. A musical composition for one voice or instrument (with or without accompaniment)

3. A flight in which the aircraft pilot is unaccompanied