The external complex ac susceptibility ${\mathrm{\ensuremath{\chi}}}_{\mathrm{e}}$=${\mathrm{\ensuremath{\chi}}}_{\mathrm{e}}^{\ensuremath{'}}$-j${\mathrm{\ensuremath{\chi}}}_{\mathrm{e}}^{\mathrm{\ensuremath{''}}}$ of a spherical ${\mathrm{Nd}}_{2}$${\mathrm{Fe}}_{14}$B single crystal has been measured as a function of temperature, field amplitude, and frequency in both directions, parallel and perpendicular to the c axis. ${\mathrm{\ensuremath{\chi}}}_{\mathrm{e}}^{\ensuremath{'}}$(\ensuremath{\perp}) for Tg${\mathrm{T}}_{\mathrm{s}}$, the spin reorientation temperature, is due to domain-magnetization rotations (DMR's). ${\mathrm{\ensuremath{\chi}}}_{\mathrm{e}}^{\ensuremath{'}}$(\ensuremath{\perp}) for T${\mathrm{T}}_{\mathrm{s}}$ is due to comparable DMR's and domain-wall displacements (DWD's), ${\mathrm{\ensuremath{\chi}}}_{\mathrm{e}}^{\ensuremath{'}}$(\ensuremath{\parallel}) for the whole range is mainly due to DWD's. DWD's are usually enhanced by DMR's. There is an anomalous sharp ${\mathrm{\ensuremath{\chi}}}_{\mathrm{e}}^{\ensuremath{'}}$(\ensuremath{\parallel}) peak at T=${\mathrm{T}}_{\mathrm{s}}$ for low fields and frequencies, which is ascribed to a technical magnetization mechanism when the first anisotropy constant equals zero. For data analyses and explanation, relations between the low-field susceptibility and the anisotropy constants, expressions of the domain-wall energy density, and a formula for ${\mathrm{\ensuremath{\chi}}}_{\mathrm{e}}^{\mathrm{\ensuremath{''}}}$ arising from eddy currents for a metallic magnetic sphere have been derived.