one, one radical (no.1)
一 represents the number one, the simplest unit. The radical sense is the same single horizontal stroke used as a component in other characters.
one; 1
one
(doing) together
number one; first; first place
one
general; universal
one part; one portion; one section
one (esp. of two); the other; one way; the other way; one direction; the other direction; one side; the other side; one party; the other party
one cup (of); one glass (of); one bowl (of); cupful; glassful; bowlful; spoonful
only; sole; unique
(what) the heck; (why) in the world; (who) on earth; (where) the hell; (how) the hell
look; glance; sight; having a look at; looking over; glancing through; running one's eyes over
all; everything; entirety; the whole
one person
whole life; a lifetime; all through life
one o'clock
instant; moment; for an instant
more or less; though not quite satisfactorily; after a fashion; pretty much; roughly; so far as it goes; technically
unity; consolidation; uniformity; unification; compatible
to fix; to settle; to standardize; to regularize
in one go; in one gulp; in one breath; without stopping; without pausing; without a rest; in one sitting; at a stretch
single word; a few words; brief comment
once more; again
agreement; accord; correspondence; consistency; coincidence; match
一 is a simple ideograph representing the number one with a single horizontal stroke.
A single horizontal line is the simplest way to write the number one.
For イチ, imagine an itchy single mosquito bite: itchy -> イチ, and one bite is all it takes.