It is the unusual accumulation of liquid/fluid in the ankles, feet and legs, which is known as peripheral oedema.
1. Heart, kidney and liver failure
Leg swelling can be a sign of heart, kidney and liver failure, due to the fact that these conditions cause the body to retain too much fluid. You should contact your doctor immediately if your legs are swollen and you also have one or more of the following symptoms: shortness of breath, tight chest, abdominal swelling, severe nausea, vomiting, abdominal discomfort, decreased urine output and a fever.