Mould
What it really means
Black or green spots of fungus that grow in damp places — around windows, in bathrooms, behind wardrobes. It can affect health, so report it to your landlord in writing. British spelling is "mould"; Americans write "mold".
Context, tips & common mistakes
British homes grow mould readily, and the first question is cause. Condensation mould responds to habits the UK climate forces on everyone: ventilate while cooking and showering, dry clothes with airflow (or outside), keep some background heat in cold weather, and leave a gap behind furniture on external walls. Wipe small patches with appropriate cleaner before they spread.
If mould persists despite reasonable habits, or tracks a leak, broken extractor fan, or cold, damp walls, that points to a building problem — report it in writing with photos and keep reporting. Landlords must deal with damp and mould hazards, and persistent black mould is a genuine health issue, especially for chests and asthma; the council's environmental health team is the escalation route if reports are ignored.