transient
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A. adj.

    1. a. Passing by or away with time; not durable or permanent;
temporary, transitory; esp. passing away quickly or soon, brief,
momentary, fleeting; spec. in Electr. (cf. sense B. 3 below).

    b. transient equilibrium (Nuclear Sci.), the condition in which the
half-life of a parent isotope is greater than that of the daughter but
comparable to the period of observation, so that after an initial
increase the total radioactivity decays with the parent's half-life and
the ratio of parent atoms to daughter atoms remains constant.

    2. Passing out or operating beyond itself; transitive; opposed to
immanent. (Often spelt transeunt for distinction from sense 1.)

    3. Passing or flowing through; passing from one thing or person to
another. Now rare.

    4. Passing through a place without staying in it, or staying only
for a short time; in quot. 1731 of birds, migratory; spec. (U.S. colloq.)
applied to a guest at a hotel, etc. (often ellipt. as n.: see B. 2).
Also transf., for transient guests, short-stay.

    5. Mus. Introduced in passing, as a note, chord, etc. not belonging
to the harmony, or to the key, of the passage; passing.

    6. U.S. (Esp. of printed matter) occasional, isolated, individual.

    B. n.

    1. A transient thing or being; something passing or transitory, not
permanent.

    2. colloq. (orig. U.S.). A person who passes through a place, or
stays in it only for a short time; spec. a `transient guest' at a hotel
or boarding-house. Also, a traveller, a tramp, a migrant worker.

    3. Physical Sci. A transient variation in current or voltage, or in
any waveform; a transient condition.

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Webster's:

1. tran.sient \-*nt\ aj [L transeunt-, transiens, prp. of transire to go 
   across, pass], fr. trans- + ire to go - more at ISSUE 1a: passing esp. 
   quickly into and out of existence : TRANSITORY, SHORT-LIVED 1b: passing 
   through or by a place with only a brief stay or sojourn 2: affecting 
   something or producing results beyond itself SYN syn TRANSITORY, EPHEMERAL, 
   MOMENTARY, FUGITIVE, FLEETING, EVANESCENT: TRANSIENT applies to what is 
   short in duration and passes quickly; TRANSITORY suggests the inevitability 
   of changing, ending, or dying out; EPHEMERAL implies striking brevity of 
   life or duration; MOMENTARY suggests coming and going quickly and being 
   therefore merely a brief interruption of a more enduring state; FUGITIVE 
   and FLEETING imply passing so quickly as to make apprehending difficult; 
   EVANESCENT suggests vanishing almost as it comes and may connote an airy or 
   fragile quality - tran.sient.ly av
2. transient n 1: one that is transient : as 1a: a transient guest 1b: a 
   person traveling about usu. in search of work 2a: a temporary oscillation 
   that occurs in a circuit because of a sudden change of voltage or of load 
   2b: a transient current or voltage