In Indo-European studies, the term s-mobile designates the phenomenon where a PIE root appears to begin with an *s- which is sometimes but not always present.
Both variants existed side by side in PIE, with Germanic preserving the forms as *steuraz and *þeuraz respectively, but Italic, Celtic, Slavic and others all having words for 'bull' which reflect the root without the *s. Compare also: Gothic stiur, German Stier, Avestan staora (cattle); but Old Norse þjórr, Greek tauros, Latin taurus, Old Church Slavonic turъ, Lithuanian tauras, Welsh tarw, Old Irish tarb, Oscan turuf, and Albanian taroç.
For example, by Grimm's law PIE *p becomes Proto-Germanic *f, but the combination *sp is unaffected by this.
Typical combinations are with voiceless stops: *(s)p-, *(s)t-, *(s)k-; with liquids and nasals: *(s)l-, *(s)m-, *(s)n-; and rarely, *(s)w-.
A number of roots beginning in *sl-, *sm-, *sn- look as if they had an s-mobile but the evidence is inconclusive, since several languages (Latin, Greek, Albanian) lost initial s- before sonorants (l, m, n) by regular sound change.