An irregular variable is a type of variable star

Variable star

A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...

 in which variations in brightness show no regular periodicity

Frequency

Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

. There are two main sub-types of irregular variable: eruptive and pulsating.

Eruptive irregular variables are divided into three categories:

• Group I variables are split into subgroups IA (spectral types O to A) and IB (spectral types F through M).

• Orion variables, GCVS type IN (irregular and nebulous), indigenous to star-forming regions, may vary by several magnitudes with rapid changes of up to 1 magnitude in 1 to 10 days, are similarly divided by spectral type into subgroups INA and INB, but with the addition of another subgroup, INT, for T Tauri stars, or INT(YY) for YY Orionis stars.

• The third category of eruptive irregulars are the IS stars, which show rapid variations of 0.5 to 1 magnitude in a few hours or days; again, these come in subgroups ISA and ISB.

Pulsating irregular giants or supergiants, called slow irregular variable

Slow irregular variable

A slow irregular variable is a variable star that exhibit no or very poorly defined periodicity in their slowly changing light emissions. Most of them are late spectral types K and M, S stars or carbon stars...

s, are all of late spectral types (K, M, C, or S), and classed as type L-LB for giants and LC for supergiants. How many of these are actually semi-regular variables that simply need more study, remains unclear.