It's inflorescence, but spike is also a type of inflorescence. The proper definition of a spike is an long indeterminate(bottom blooms open first) inflorescence with sessile (no peduncle) flowers. So we even mis-use the term since many orchids are pedunculated. An elongated inflorescence with pedunculated flowers is called a raceme (phals and large bloomed oncs are in this category). Oncidiums like the twinkle types that have mini spikes things branching off the main spike are compound racemes, or panicles type inflorescences.
Somehow saying "My phal is spiking!!" sounds so much more exciting than saying "my phal is producing an inflorescence!" So I'll stick to the improper use of spike!