This is an increased or decreased need to drink water.
The cat either drinks more or less water than is normal for him. Other symptoms include an increased of decreased need to urinate.
It can be a symptom of cystitis tapeworm infestation, diabetes insipidus or diabetes mellitus.
What to do
Keep an eye on urine deposits in the litter tray; if you know what is generally normal for your pet, then a change will be detected early. Cystitis is indicated by discomfort and straining to urinate, while tapeworm infestation is signalled by the visible presence of worms in faeces, and also tiny white segments of them sticking to fur around the anus.
Take your cat for a veterinary examination: treatment will depend on the cause of the condition. If tapeworms are to blame, deworming will be in order, while cystitis is treatable with antibiotics.