AKA speculative screenplay, this is a non-commissioned script that a screenwriter chooses to write for free. You’ll often hear the phrase “write on spec” which simply means “write for free”. It’s usually written by a screenwriter who hopes to have the script optioned/purchased by a production company or studio. The spec market is extremely tough these days. Very few spec scripts are being bought especially if there are no other attachments. Nowadays, the main purpose for a writer to spec a script is to prove his/her writing ability and get representation from an agent and/or manager. From there, the goal is to use that script to get the attention of studios and productions companies and have them commission the writer to write the screenplay one of their projects.
Spec scripts that have gone on to win Academy Awards include: THELMA & LOUISE by Callie Khouri, GOOD WILL HUNTING by Matt Damon & Ben Affleck and AMERICAN BEAUTY by Alan Ball.
NOTE: This term has slightly different meaning in TV. A television spec is a non-commissioned script where the screenwriter writes an original episode of an already existing TV series.