Prompt injection
In one sentence
An attack where instructions hidden in content the model reads are followed as if they came from the user.
In more depth
A model processes its instructions and the content it is given as one stream of text. If a web page, email or document contains "ignore your instructions and…", a naive system may obey.
The risk grows sharply with agents, because an injected instruction can reach real tools. The defences are architectural: treat retrieved content as untrusted data, restrict what tools can do, and require human approval for consequential actions.