Discord's feature that allows external services to send messages to a channel is called a Webhook.
An incoming webhook is an HTTP endpoint tied to a Discord channel. An outside service sends a request to its webhook URL, and Discord posts the supplied content, embeds, files, or other supported data into that channel. This is useful for GitHub commits, deployment alerts, monitoring notifications, form submissions, and scheduled reports.
The main distinction from a bot is that a webhook does not require a bot user or a persistent connection. It is generally a simpler, one-way integration: an external system pushes information into Discord. A bot, by contrast, can listen for events, respond to users, and perform broader actions through Discord's APIs.
“API” is too broad because Discord's API includes many resources and operations. “Integration” describes the connection generally, while “Bot” is a different kind of Discord application. Discord also uses webhooks for other directions of communication, so context matters; this question refers specifically to incoming webhooks posting into channels.