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The preferred term is "soaring," since this is what all sailplane pilots seek to do. Technically, soaring is the act of maintaining or gaining altitude in unpowered flight (aided by rising currents of air). It's interesting to note that gliding is always occurring even while soaring — the pilot is going up in air rising faster than the plane is gliding down.
For the aircraft themselves, the situation is somewhat reversed. The common term is "glider" (even for sailplanes, which are considered the higher-performance end of the spectrum).