Martymachlia is a fetish that involves arousal stemming from others watching their sexual activity. This falls under the umbrella term "exhibitionism." However, it is its own paraphilia distinct in that it does not involve many of the other turn ons that can come with exhibitionism such as flashing or streaking. Martymachlia is specific to sexual activity that is witnessed by others.
Martymachlia can be satisfied in a number of ways including inviting additional parties to watch one's sexual activity, engaging in sexual activity at a party or other venue where other people are present, using a video camera, or positioning oneself in front of a window or other place that makes it likely one will be seen.
It is important to remember that the people see the martymachiliacs should be willing participants who are consenting to what the are seeing.