Instant messaging (IM) technology is a type of online chat allowing real-time text transmission over the Internet or another computer network. Messages are typically transmitted between two or more parties, when each user inputs text and triggers a transmission to the recipient(s), who are all connected on a common network. It differs from email in that conversations over instant messaging happen in real-time (hence "instant"). Most modern IM applications (sometimes called "social messengers", "messaging apps" or "chat apps") use push technology and also add other features such as emojis (or graphical smileys), file transfer, chatbots, voice over IP, or video chat capabilities.

Instant messaging systems tend to facilitate connections between specified known users (often using a contact list also known as a "buddy list" or "friend list"), and can be standalone applications or integrated into e.g. a wider social media platform, or a website where it can for instance be used for conversational commerce. IM can also consist of conversations in "chat rooms". Depending on the IM protocol, the technical architecture can be peer-to-peer (direct point-to-point transmission) or client–server (an IM service center retransmits messages from the sender to the communication device). It is usually distinguished from text messaging which is typically simpler and normally uses cellular phone networks.

Instant messaging applications can store messages with either local-based device storage (e.g WhatsApp, Viber, Line, WeChat, Signal etc.) or cloud-based server storage (e.g Telegram, Skype, Facebook Messenger, Google Meet/Chat, Discord, Slack etc.).

Instant messaging was pioneered in the early Internet era; the IRC protocol was the earliest to achieve wide adoption. Later in the 1990s, ICQ was among the first closed and commercialized instant messengers, and several rival services appeared afterwards as it became a popular use of the Internet. Beginning with its first introduction in 2005, BlackBerry Messenger, which initially had been available only on BlackBerry smartphones, soon became one of the most popular mobile instant messaging apps worldwide. BBM was for instance the most used mobile messaging app in the United Kingdom and Indonesia. Instant messaging remains very popular today; IM apps are the most widely used smartphone apps: in 2018 there were over 50 million Signal users, 980 million monthly active users of WeChat and 1.3 billion monthly users of WhatsApp Messenger.