bar chart vs histogram

bar chart and histogram both are nouns.

Word	Noun	Adjective	Verb	Adverb
bar chart	Yes	No	No	No
histogram	Yes	No	No	No

As nouns, histogram is a hyponym of bar chart; that is, histogram is a word with a more specific, narrower meaning than bar chart:

bar chart: a chart with bars whose lengths are proportional to quantities
histogram: a bar chart representing a frequency distribution; heights of the bars represent observed frequencies

bar chart (noun)	histogram (noun)
a chart with bars whose lengths are proportional to quantities	a bar chart representing a frequency distribution; heights of the bars represent observed frequencies