One fixture or the whole property?
Flicker isolated to one fixture often relates to the lamp, the fixture or its dimmer. Flicker across multiple rooms, or in step with large equipment starting, points to something shared — a circuit, a connection, the panel or the service.
Frequent causes
Incompatible dimmers and LED lamps, loose connections at a device or splice, overloaded circuits, and conditions at the panel or utility service are the usual explanations.
When to call
Widespread flicker, flicker with heat or odor, and flicker that appeared suddenly are worth prompt evaluation because loose or failing connections can generate heat.