What is Equivalence, equivalent volume?

Example 36.6 (Permanganate, its own indicator)

The permanganate ion $\ce{MnO4^-}$ is deep purple, and the manganese ions $\ce{Mn^{2+}}$ it gives are almost colourless; the iron ions give the solution only a pale yellow tint. Before equivalence, each purple drop is decolourised as it falls; the first drop that is not marks equivalence: the flask turns pale pink and stays so.

Example 36.7 (Vitamin C and iodine)

Vitamin C, ascorbic acid $\ce{C6H8O6}$ ($M = 176.0\,\mathrm{g}/\mathrm{mol}$), is a reductant; iodine $\ce{I2}$ oxidises it:

$10.0\,\mathrm{mL}$ of orange juice, with a little starch, are titrated by an iodine solution at $5.00 \times 10^{-3}\,\mathrm{mol}/\mathrm{L}$. Before equivalence the iodine is used up; the first drop in excess turns the starch dark blue. If $V_E = 5.70\,\mathrm{mL}$, the juice held $n = 5.00 \times 10^{-3} \times 5.70 \times 10^{-3} = 2.85 \times 10^{-5}\,\mathrm{mol}$ of vitamin C, that is $5.0\,\mathrm{mg}$ in $10.0\,\mathrm{mL}$: $0.50\,\mathrm{g}/\mathrm{L}$.