time slot: a time assigned on a schedule or agenda
time interval: a definite length of time marked off by two instants
protoctista: in most modern classifications, replacement for the Protista; includes: Protozoa; Euglenophyta; Chlorophyta; Cryptophyta; Heterokontophyta; Rhodophyta; unicellular protists and their descendant multicellular organisms: regarded as distinct from plants and animals