In policy debate, a disadvantage (abbreviated as DA, and sometimes referred to as: Disad) is an argument that a team brings up against a policy action that is being considered.
A Disadvantage usually has four key elements. These four elements are not always necessary depending on the type of disadvantage run, and some are often combined into a single piece of evidence. A Unique Link card, for example, will include both a description of the status quo and the plan's effect on it. A traditional threshold DA, however, has a structure as follows:
Uniqueness shows why the Impacts have not occurred yet or to a substantial extent and will uniquely occur with the adoption of either the Affirmative's plan or the Negative's counterplan.
Example: If the negative team argues that the affirmative plan will result in nuclear proliferation, it would also argue that the status quo will avoid nuclear proliferation. If the Affirmative claims that nuclear proliferation is already occurring, the negative team could argue that adoption of the plan would result in a unique increase in nuclear proliferation. If the plan causes no net change in the rate of nuclear proliferation, the disadvantage is not unique to the plan, and therefore not relevant.
For the disadvantage to have relevance in the round, the negative team must show that the affirmative plan causes the disadvantage that is claimed. If the DA stated that the plan takes money from the government, and the affirmative team shows that the plan does not increase governmental spending, then the DA would be considered to have "no link".
The internal link connects the link to the impact, or, it shows the steps the link causes to get to the impact. Not all DA's use an internal link but some have multiple internals. The internal link in our example would be that government spending leads to economic collapse.
The impact is the result of the policy action that make it undesirable. These results are at the end of the chain of reasoning of your DA (starts with your link with internal links spanning over the Brink with Uniqueness and lead to the Impact), then continuing along with the example, an impact would be that economic collapse may cause nuclear war. The Impact is the edge of the sword of your DA and is usually a significantly bad event caused by inertia evident through the internal links inside the link off over the brink and uniquely so.
Internal links are often undesirable things by themselves, and could be considered impacts. However, the worst of the consequences, or the final one in the chain of events, is usually given the label of "impact". For example, nuclear war is probably worse than economic collapse, so nuclear war is given the "impact" label, even though economic collapse (the internal link) could itself be viewed as an impact.