demerger & divestiture

demerget is opposite of merger. in merger two cos merge and new co is formed. whereas in demerger a co division is separated from existing co and new co is formed. say for e.g a co has two division and one of them is earning good profits then the co separates that division and new co is formed.
whereas in divestiture the investment made in another co is being sold to lower it's stake

is slump sale also considered as divestiture..?

no it's not

means.. particularly withdrawing of investment ( share transfer ) is called as divestiture or disinvestment or divestment..

And sale of any asset/ undertaking is not considered as Divestiture whether slump or otherwise..?

yes that's correct