ka'o zei namcu zei-lujvo

x1 is a complex number (understood as such).

Not necessarily strictly imaginary (or real, for that matter). This word relies on xo'ei to implicitly fill both sides of ka'o. See also: ka'o'ai zei namcu, noka'ono (with PA neither immediately before nor immediately after it), nozeika'o zei namcu, and ka'ozeino zei namcu.

On affix form:

kanro
x1 is healthy/fit/well/in good health by standard x2.
namcu
x1 (li) is a number/quantifier/digit/value/figure (noun); refers to the value and not the symbol.
zekri
x1 (event/state) is a punishable crime/[taboo/sin] to people/culture/judges/jury x2.

In definition:

cmapagjvo
x1 (text) is a lujvo with meaning x2 and arguments x3, and which is constructed from tanru/veljvo x4 such that each element of the veljvo is ultimately only a cmavo (including "{zei}", "{ke}"/"{ke'e}", "{bo}", etc.), gismu, or rafsi of such cmavo or gismu.
zejvo
x1 (text) is a Lojban compound predicate using the particle “zei”; with meaning x2, arguments x3, built from metaphor (tanru) x4.
luvrzei
x1 (text) is a Lojban compound predicate using the particle “zei”; with meaning x2, arguments x3, built from metaphor (tanru) x4.
zeiljvo
x1 (text) is a Lojban compound predicate using the particle “zei”; with meaning x2, arguments x3, built from metaphor (tanru) x4.
zo si si zei fa'o
x1 is a nonsense zei-lujvo using parts x2, silly by standard x3.

In notes:

lujna'u
x1 is a complex number.
tolxarna'u
x1 is a real number.
kai'o (exp!)
quaternion i
ka'o'ai (exp!)
imaginary i (non-comma)
ka'o'ei (exp!)
imaginary i, comma - spherical coordinates: first coordinate gives magnitude (complex modulus/radius) of the number, the second number gives the angle from the positive real axis measured counterclockwise toward the 'positive' imaginary axis (default: in the primary branch/Arg) as measured in some units (which that number should contain; the contextless default will suppose radians); the angle is not normalized.
kau'o (exp!)
generic algebra unit en
kei'o (exp!)
quaternion j
koi'o (exp!)
quaternion k
ka'omcu
x1 is a complex number with real part x2 and imaginary part x3
epsi bu
digit/number: dual ε of dual number in linear algebra; square root of 0
ka'ozeino zei namcu
x1 is a pure real number (such that its imaginary part is 0) understood as an example of a complex number.
nozeika'o zei namcu
x1 is a pure imaginary number (such that its real part is 0) understood as an example of a complex number.