Alert is a colloquial term used to define a machine-to-person communication that is important and/or time sensitive. An alert contains user-requested content such as a reminder (important), a notification (urgent), and ultimately an alert (important and urgent). Alert messaging or alert notification is the delivery of alerts to recipients.

Alert messaging emerged from the study of Personal Information Management (PIM),^{[citation needed]} the science of discovering how people perform certain tasks to acquire, organize, maintain, retrieve and use information relevant to them. Alert notification is a natural evolution of the concept of RSS^{[citation needed]} which makes it possible for people to keep up with web sites in an automated manner. Alerting makes it possible for people to keep up with the information that matters most to them.

Alerts are typically delivered through a notification system and the most common application of the service is machine-to-person communication. Very basic services provide notification services via email or SMS. More advanced systems (for example AOL) provides users with the choice of selecting a preferred delivery channel such as e-mail, Short Message Service (SMS), instant messaging (IM), via voice through voice portals and more. Novel approaches provide users with the ability to schedule their own alerts (for example Google Calendar). The most sophisticated service providers embrace all capabilities, aggregating a multitude of reminder, notifications and alert, catering the delivery system to the specific context of the content being delivered thus enabling users to create sophisticated scenarios.

A busy signal (or busy tone or engaged tone) in telephony is an audible or visual signal to the calling party that indicates failure to complete the requested connection of that particular telephone call.

There are several distinctly different types of busy signals:

Many different countries have different signalling tones that act as "busy signals".