Anything undeclared isn't stated or admitted to in a public way. An undeclared political candidate may be seriously considering running for President, but she hasn't officially announced it.
An undeclared war is conflict and violence that affects people's lives despite the fact that no government is officially calling it a war. If you're a college student with an undeclared major, you haven't decided which subject to focus on. Once you choose, you'll have a declared major. When it comes to paying taxes, any income that someone hides from the IRS is also undeclared: "She had to pay a fine because of those undeclared tips."