This refers to a sexual relationship between an adult male and a pubescent or adolescent male. It’s different than pedophilia, as that specifically refers to a pre-pubescent male or female.
The difference seems to hinge on age.
Why I Looked It Up
In a discussion of Horatio Algers, it was said he had to leave his position due to “charges of pederasty.”
His Wikipedia page a little more explicit:
Early in 1866, a church committee of men was formed to investigate reports that Alger had sexually molested boys. Church officials reported to the hierarchy in Boston that Alger had been charged with “the abominable and revolting crime of gross familiarity with boys”
Apparently the investigation was started by the accusations of a 13-year-old boy. This would likely fit the definition of pederasty, rather than pedophilia.