decellaration and retardation

deceleration has no meaning... further.. negative acceleration may not be deceleration... deceleration means decrease in speed.. so if u take +ve x-axis as +ve direction... then a body going towards right has velocity +ve... now if a force acts on it towards left then acceleration produced is negative... till some time its speed reduces to zero.. (til this much it can be called deceleration) but after that its speed again increases in the -ve x-direction... but according to our coordinates acceleration is still negative but here there is no deceleration.... hence it is best to always use "acceleration"

if u analyse the meaning then they are both same though