one machine; one device
Used when counting machines, devices, or mechanical equipment with the counter 機.
There is one large press machine in this factory.
one aircraft; one plane
Used when counting aircraft, helicopters, or similar flying vehicles.
One passenger plane landed at the airport.
one life; one remaining life
Video game term for a single life or remaining life count.
I challenge the boss with only one life left.
一気 (いっき) means 'in one go/without stopping', while 一機 (いっき) is unrelated and means 'one aircraft'.
Standard kanji spelling for counting machines or aircraft.
Numerical variant; common in technical or informal writing.
Compound of 一 (one) and the counter 機 (machine, aircraft). The reading いっき is a regular euphonic change from いち + き.