The phrase describing the grim condition was used in court papers filed by members of the Mandela family - according to lawyers cited in media reports - who are feuding with the former president's grandson, Mandla Mandela.
"The doctors deny that the former president is in a vegetative state," Zuma's office said.
Zuma, using Mandela's clan name, had said after visiting the hospital Thursday afternoon: "Madiba is receiving the best medical care from a multi-disciplinary team of health professionals who are at his bedside around the clock."
Mandela's family, in court documents submitted last week, indicated that the first black South African president was on life-support machines to aid his breathing.
His wife, Graca Machel, in her first public comments on Mandela's condition, seemed more upbeat Thursday.
"Although Madiba may sometimes be uncomfortable, very few times he is in pain, but he is fine," said Machel, while issuing a call for "unity."
The 94-year-old Nobel peace laureate was admitted to hospital on June 8 for a recurring lung infection, thought to be related to the damage inflicted on his lungs during his 27 years as a political prisoner under the apartheid regime.
Last week, Zuma canceled a trip to Mozambique, as it appeared Mandela's condition had taken a turn for the worse. A day later, the presidency said the liberation hero's condition had stabilized.
Meanwhile, the Mandela family is being rocked by internal disputes over where to bury Nelson Mandela when he dies.
The remains of three of Mandela's children were reburied in their original resting place in Qunu, the family's ancestral village, after they were exhumed Wednesday from the village of Mvezo.
The remains were moved in 2011 from Qunu to Mvezo by Mandla Mandela, the former president's grandson, apparently without permission from the rest of the family. The family has charged he carried out the move in the hope that Nelson Mandela would also be buried there, making Mvezo a tourist attraction.
It is believed Nelson Mandela has expressed his wish to be buried near other family members in Qunu in the Eastern Cape province, in South Africa's remote, rural south-east.
At the same time, there are ongoing financial disputes within the family over control of the Mandela trust. The various disagreements have split the family.
The mud-slinging between family members - including public allegations of infidelity and children born out of wedlock - has sparked concern in South Africa.
Chief Phathekile Holomisa, head of the Council of Traditional Leaders, an influential forum, went on local television to urge the family to sit down for talks.
"My appeal would be to all of them ... that they must go back home and talk among themselves as a family," he told broadcaster Enca.
The call followed on the heels of public reprimands by leaders of the governing African National Congress (ANC).
"They need to take a chapter out of Madiba's life and learn how to find peace against all odds," ANC Secretary General Gwede Mantashe said this week. "This ability is the reason why Nelson Mandela and other comrades were able to deliver our freedom."
