hsla()

Colors can be defined in the hue-saturation-lightness-alpha model (HSLa) using the hsla() functional notation. HSLa extends the HSL color model to include the alpha channel, allowing specification of the opacity of a color.
a means opacity: 0=transparent; 1=opaque;

hsla(240,100%,50%,0.05)   /* 5% opaque blue */   
hsla(240,100%,50%, 0.4)   /* 40% opaque blue */  
hsla(240,100%,50%, 0.7)   /* 70% opaque blue */  
hsla(240,100%,50%,   1)   /* full opaque blue */