Wow and Flutter
Digital audio is precisely clocked, so that sounds play back exactly as they were recorded. However, since analog machines playback from tape being pulled across a head by a mechanical shaft (called a capstan) that's coupled with a rubber wheel called either a pinch or pressure roller, there are often subtle variations in the pitch of the sound. Phonographs also have these same variations due to mechanical issues.
Higher frequency, smaller-deviations from the true pitch are called "flutter", and lower-frequency larger-deviations are called "wow". Wow is generally perceived as a pitch change, while flutter is often perceived as timbral degradation, since unwanted inharmonic overtones are produced.
On tape recorders, often simply changing the diameter of the supply hub can improve the speed consistency, since the hold-back tension increases as the diameter of the supply hub decreases.
Also, just like a violin bow on a string, the tape can actually bounce on the head due to the friction, and produce what's known as "scrape flutter". To minimize this, most tape decks have one or more rollers close to the heads called "scrape filters".
In addition, keeping the heads, guides, capstan and pressure roller clean and making sure that the tape is free of archival shed will also help decrease the wow and flutter.