Botnet
A network of hijacked internet-connected devices that an attacker controls remotely, usually without the owners noticing.
A botnet is a crowd of compromised machines working for someone who does not own them. Attackers scan the internet for devices with known holes, take them over quietly, and add them to a pool they can command all at once. The classic recruits are not laptops but forgotten gadgets: old routers, security cameras, network drives, anything that stopped receiving security updates years ago and still sits online.
Once assembled, a botnet is rented out or used to knock services offline, send spam, mine cryptocurrency, or hide the source of other attacks. Owners rarely notice, because a device in a botnet keeps doing its normal job. The practical defence is unglamorous: replace or disconnect hardware that no longer gets updates, and change default passwords on anything that is reachable from the internet.