Definition
A necessary condition is one that must hold for an outcome to occur — remove it and the outcome becomes impossible. A sufficient condition is one whose presence guarantees the outcome on its own. The two are different, and confusing them is a common reasoning error.
Oxygen is necessary for a fire but not sufficient: air alone does not ignite. Striking a match in the right conditions may be sufficient for a flame, but it is not necessary, since other ignition sources exist. A condition can be necessary, sufficient, both, or neither.
Why it matters
How it works
When analyzing what causes an outcome, ask of each candidate condition two separate questions. Could the outcome happen without it? If not, the condition is necessary. Does its presence alone guarantee the outcome? If so, it is sufficient.
Most real success requires assembling a full set of necessary conditions, none of which is sufficient alone. Identifying every one of them — and noticing which is missing — is often the whole task.