A rest is an interval of silence in a piece of music, marked by a sign indicating the length of the pause. Each rest symbol corresponds with a particular note value:

The combination of rests used to mark a pause follows the same rules as for notes. For more details see note value.

longa (or four-measure rest)
double whole rest / breve rest
whole rest / semibreve rest
half rest / minim rest
quarter rest / crotchet rest
eighth rest / quaver rest
sixteenth rest / semiquaver rest
thirty-second rest / demisemiquaver rest
sixty-fourth rest / hemidemisemiquaver rest Multiple measure rests
In instrumental parts, rests of more than one measure in the same meter and key may be indicated with a multiple measure rest, showing the number of measures of rest, as shown. Multiple measure rests of variable duration are usually drawn in one of two ways: either as long, thick horizontal lines placed on the middle line of the staff, with serifs at either end, or as thick diagonal lines placed between the second and fourth lines of the staff. They denote a silence several times the duration of a whole rest.
The number of whole rest lengths for which the multiple measure rest lasts is indicated by a number printed above the musical staff (usually at the same size as the numerals in a time signature). Where the silence is for less than eight whole rest lengths, some publishers use a combination of four measure rests, double whole rests and whole rests to graphically indicate the extent of the rest. This serves as a counting aid and derives from 19th-century notation conventions. If a meter or key change occurs during a multiple-measure rest, the rest must be broken up as required for clarity, with the change of key and/or meter indicated between the rests. This also applies in the case of a double-barline, which demarcates musical phrases or sections (a tacet instrumental part to a song may contain a sequence of multiple eight-measure rests, for instance).
The four-measure rest or longa rest is a symbol found in Western musical notation denoting a silence four times the duration of a whole rest. They are only used in long silent passages which are not divided into bars.
The two-measure rest or breve rest is another symbol found in Western musical notation denoting a silence twice the duration of a whole rest. They are usually found in conjunction with the aforementioned four-measure rest.