a mechanic in Fate Core representing an extremely transient sorta-but-not-quite aspect.
Boosts (Fate SRD) are an extremely transient feature that's essentially a free invoke with its own name.
Ryan Macklin went into more detail on the behaviour and design process behind boosts in his blog: Fate Boosts Revisited. The important clarifying parts have been added to the Fate SRD inside the boosts section. Among the clarifications: they're not full aspects, and can't be compelled or have fate points spent on them.